Why are lenses characterized by focal length and not the effective field of view?

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In photography, it's the accepted norm to say that if you know the sensor size of your camera and the focal length of your lens, then you know the field of view of your system.



However, in optics, it's well known that for a fixed sensor size, the distance from the lens to the image plane (which we can equate with the flange distance) also affects your field of view. Everyone who mounted hisher lens on a macro adapter had seen that increasing the flange distance will narrow down your field of view, and they will end up with a macro lens.



So my question is why do we characterize lenses with the focal length and not the effective field of view?



I realize that it could be the case that it was just an arbitrary historical decision, but I'm curious whether there's justification for that which I'm missing.










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    In photography, it's the accepted norm to say that if you know the sensor size of your camera and the focal length of your lens, then you know the field of view of your system.



    However, in optics, it's well known that for a fixed sensor size, the distance from the lens to the image plane (which we can equate with the flange distance) also affects your field of view. Everyone who mounted hisher lens on a macro adapter had seen that increasing the flange distance will narrow down your field of view, and they will end up with a macro lens.



    So my question is why do we characterize lenses with the focal length and not the effective field of view?



    I realize that it could be the case that it was just an arbitrary historical decision, but I'm curious whether there's justification for that which I'm missing.










    share|improve this question

























      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      In photography, it's the accepted norm to say that if you know the sensor size of your camera and the focal length of your lens, then you know the field of view of your system.



      However, in optics, it's well known that for a fixed sensor size, the distance from the lens to the image plane (which we can equate with the flange distance) also affects your field of view. Everyone who mounted hisher lens on a macro adapter had seen that increasing the flange distance will narrow down your field of view, and they will end up with a macro lens.



      So my question is why do we characterize lenses with the focal length and not the effective field of view?



      I realize that it could be the case that it was just an arbitrary historical decision, but I'm curious whether there's justification for that which I'm missing.










      share|improve this question















      In photography, it's the accepted norm to say that if you know the sensor size of your camera and the focal length of your lens, then you know the field of view of your system.



      However, in optics, it's well known that for a fixed sensor size, the distance from the lens to the image plane (which we can equate with the flange distance) also affects your field of view. Everyone who mounted hisher lens on a macro adapter had seen that increasing the flange distance will narrow down your field of view, and they will end up with a macro lens.



      So my question is why do we characterize lenses with the focal length and not the effective field of view?



      I realize that it could be the case that it was just an arbitrary historical decision, but I'm curious whether there's justification for that which I'm missing.







      focal-length field-of-view






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      edited 3 hours ago









      scottbb

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      asked 12 hours ago









      Yuval Weissler

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          5 Answers
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          up vote
          4
          down vote













          This question is predicated on a misconception. The flange distance is included in the focal length in lens labels — it is the distance from the optical center of the lens focused at infinity to the imaging medium. This includes the flange distance. (See What is the reference point that the focal length of a lens is calculated from?, and What exactly is focal length when there is also flange focal distance?)



          This means that for a given sensor size, lenses made for different mounts are still comparable — a 24mm lens gives (approximately) the same field of view regardless of the mount distance of any given system. So, focal length does correspond to field of view.



          You note that using a macro adapter affects field of view. In fact, they also increase the focal length, although since macro extension tubes remove the ability to actually focus at infinity, we leave practical reality and get into the realm of theory.






          share|improve this answer






















          • Flange distance is very much independent of the effective focal length of the lens. one can design a lens to have any effective focal length one wishes, and then enclose said design in a mechanical casing which will result in any distance one wishs between the back principle plane of the lens and the sensor (which we can equate to the flange distance). the resulting magnification (and as an extension, the FOV) are derived from the above distance. this is basic geometrical optics.
            – Yuval Weissler
            11 hours ago






          • 2




            In photography, lenses are marketed and sold as I have described. You are right that they are independent, but lenses are made and sold for a particular mount, and that means the flange focal distance of that mount is included in the number on the lens.
            – mattdm
            10 hours ago










          • This means that a 24mm lens on Pentax K mount will have approximately the same field of view as a 24mm lens for Canon or Nikon or whatever else, assuming the same sensor size.
            – mattdm
            10 hours ago










          • Your note that adding a macro adapter changes the focal length is not true - at least not from the physics point of view, which is what I'm interested in. The effective focal of any lens does not change, no matter how far from the sensor you choose to place it.
            – Yuval Weissler
            8 hours ago










          • @YuvalWeissler Again, the focal length of a complex lens is measured when that lens is focused at infinity. If you change the distance from the lens to the sensor, you will need to change the focus in order to make the focal plane coincide (that is "not have everything be all blurry"). This is true from "the physics point of view" and from any other point of view.
            – mattdm
            6 hours ago

















          up vote
          2
          down vote














          So my question is why do we characterize lenses with the focal length and not the effective field of view?




          The very good reason for cameras is that field of view also strongly depends on the sensor size. A small sensor captures a more narrow view than a larger sensor can.



          The lens focal length does provide some field of view, which the sensor size crops to capture possibly a lesser amount of it. Cameras with tiny sensors only capture a small field, so they have to use a much shorter focal length lens to compare to an expected "regular picture" view seen by other cameras with larger sensors. Crop Factor compares that sensor size view to the historical 35 mm film frame size view that so many of us are very familiar with.



          Focal length is NOT at all about the mounting flange distance. The internal focus node can be moved by design. A "telephoto" lens means that node is slightly in front of the front lens element (lens is shorter than the focal length). A retro-focus (wide angle) lens places that focus node behind the rear element, to create space behind the lens. But focal length is to that focal node.



          The focal length marked on the lens is when focused at infinity. Some imagine that is the definition of focal length, but it is merely one setting for it. Zoom lenses also vary focal length.



          When focused closer than infinity, the lens is extended forward (the frontal elements are, or possibly only internal elements) so that the internal focus node in the lens is further from the sensor plane. If we instead assume focal length is the distance from this internal node to the sensor plane, then of course the focal length is a bit longer when focused closer. That longer focal length changes things, like f-number, which can affect exposure, so regular lenses don't allow distances shorter than some nominal close distance, typically at around 0.1x magnification.






          share|improve this answer





























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            Focal length and lens FOV are inherently the same thing. The variable is how much of the lens' FOV is utilized (i.e. extension tubes/TC's/crop sensor/etc). Why would you specify a lens' characteristic by something that may be variable?






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              The focal length of a single lens always stays the same, the field of view changes as you focus on something closer than infinity, that may be a good reason to specify the focal length, and not the field of view.






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              • The field of view changes because the actual focal length changes.
                – mattdm
                6 hours ago










              • @mattdm Do you have any reference for that? According to thin lens equations the image distance also changes as the subject distance changes for a single simple lens element. Therefore the field of view changes too.
                – Orbit
                6 hours ago










              • photo.stackexchange.com/questions/16549/…
                – mattdm
                5 hours ago






              • 1




                This is not hard. Focal length is to where the lens focuses behind. The focal length marked on the lens is when focused at infinity. When focused at a closer distance, the lens elements must be moved forward to still focus on the same image plane. That can be several possible longer focal lengths (over a small range), so the one at infinity is simply the one marked. Any lens formula must use the actual focal length for the actual focused distance. If at 1:1 magnification, the focal length behind is the same as the focused distance in front. Or focused at infinity is half that behind then.
                – WayneF
                1 hour ago







              • 1




                @Orbit We're talking about photographic lenses here, which are, excepting some novelty acts, complex lenses, not simple lenses, let alone an idealized thin lens.
                – mattdm
                44 mins ago

















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              I think we use focal length as attribute of lenses fro historical reasons. Time ago glass elements can be measured by distance from the focus (using Sun light) to the "center" of the element. Same is true for the lenses. To get field of view you need to do some mathematics and (maybe) this stop ordinary people (not mathematicians) to use it.



              But this question IMHO is too broad and opinion based....






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                5 Answers
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                active

                oldest

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                5 Answers
                5






                active

                oldest

                votes









                active

                oldest

                votes






                active

                oldest

                votes








                up vote
                4
                down vote













                This question is predicated on a misconception. The flange distance is included in the focal length in lens labels — it is the distance from the optical center of the lens focused at infinity to the imaging medium. This includes the flange distance. (See What is the reference point that the focal length of a lens is calculated from?, and What exactly is focal length when there is also flange focal distance?)



                This means that for a given sensor size, lenses made for different mounts are still comparable — a 24mm lens gives (approximately) the same field of view regardless of the mount distance of any given system. So, focal length does correspond to field of view.



                You note that using a macro adapter affects field of view. In fact, they also increase the focal length, although since macro extension tubes remove the ability to actually focus at infinity, we leave practical reality and get into the realm of theory.






                share|improve this answer






















                • Flange distance is very much independent of the effective focal length of the lens. one can design a lens to have any effective focal length one wishes, and then enclose said design in a mechanical casing which will result in any distance one wishs between the back principle plane of the lens and the sensor (which we can equate to the flange distance). the resulting magnification (and as an extension, the FOV) are derived from the above distance. this is basic geometrical optics.
                  – Yuval Weissler
                  11 hours ago






                • 2




                  In photography, lenses are marketed and sold as I have described. You are right that they are independent, but lenses are made and sold for a particular mount, and that means the flange focal distance of that mount is included in the number on the lens.
                  – mattdm
                  10 hours ago










                • This means that a 24mm lens on Pentax K mount will have approximately the same field of view as a 24mm lens for Canon or Nikon or whatever else, assuming the same sensor size.
                  – mattdm
                  10 hours ago










                • Your note that adding a macro adapter changes the focal length is not true - at least not from the physics point of view, which is what I'm interested in. The effective focal of any lens does not change, no matter how far from the sensor you choose to place it.
                  – Yuval Weissler
                  8 hours ago










                • @YuvalWeissler Again, the focal length of a complex lens is measured when that lens is focused at infinity. If you change the distance from the lens to the sensor, you will need to change the focus in order to make the focal plane coincide (that is "not have everything be all blurry"). This is true from "the physics point of view" and from any other point of view.
                  – mattdm
                  6 hours ago














                up vote
                4
                down vote













                This question is predicated on a misconception. The flange distance is included in the focal length in lens labels — it is the distance from the optical center of the lens focused at infinity to the imaging medium. This includes the flange distance. (See What is the reference point that the focal length of a lens is calculated from?, and What exactly is focal length when there is also flange focal distance?)



                This means that for a given sensor size, lenses made for different mounts are still comparable — a 24mm lens gives (approximately) the same field of view regardless of the mount distance of any given system. So, focal length does correspond to field of view.



                You note that using a macro adapter affects field of view. In fact, they also increase the focal length, although since macro extension tubes remove the ability to actually focus at infinity, we leave practical reality and get into the realm of theory.






                share|improve this answer






















                • Flange distance is very much independent of the effective focal length of the lens. one can design a lens to have any effective focal length one wishes, and then enclose said design in a mechanical casing which will result in any distance one wishs between the back principle plane of the lens and the sensor (which we can equate to the flange distance). the resulting magnification (and as an extension, the FOV) are derived from the above distance. this is basic geometrical optics.
                  – Yuval Weissler
                  11 hours ago






                • 2




                  In photography, lenses are marketed and sold as I have described. You are right that they are independent, but lenses are made and sold for a particular mount, and that means the flange focal distance of that mount is included in the number on the lens.
                  – mattdm
                  10 hours ago










                • This means that a 24mm lens on Pentax K mount will have approximately the same field of view as a 24mm lens for Canon or Nikon or whatever else, assuming the same sensor size.
                  – mattdm
                  10 hours ago










                • Your note that adding a macro adapter changes the focal length is not true - at least not from the physics point of view, which is what I'm interested in. The effective focal of any lens does not change, no matter how far from the sensor you choose to place it.
                  – Yuval Weissler
                  8 hours ago










                • @YuvalWeissler Again, the focal length of a complex lens is measured when that lens is focused at infinity. If you change the distance from the lens to the sensor, you will need to change the focus in order to make the focal plane coincide (that is "not have everything be all blurry"). This is true from "the physics point of view" and from any other point of view.
                  – mattdm
                  6 hours ago












                up vote
                4
                down vote










                up vote
                4
                down vote









                This question is predicated on a misconception. The flange distance is included in the focal length in lens labels — it is the distance from the optical center of the lens focused at infinity to the imaging medium. This includes the flange distance. (See What is the reference point that the focal length of a lens is calculated from?, and What exactly is focal length when there is also flange focal distance?)



                This means that for a given sensor size, lenses made for different mounts are still comparable — a 24mm lens gives (approximately) the same field of view regardless of the mount distance of any given system. So, focal length does correspond to field of view.



                You note that using a macro adapter affects field of view. In fact, they also increase the focal length, although since macro extension tubes remove the ability to actually focus at infinity, we leave practical reality and get into the realm of theory.






                share|improve this answer














                This question is predicated on a misconception. The flange distance is included in the focal length in lens labels — it is the distance from the optical center of the lens focused at infinity to the imaging medium. This includes the flange distance. (See What is the reference point that the focal length of a lens is calculated from?, and What exactly is focal length when there is also flange focal distance?)



                This means that for a given sensor size, lenses made for different mounts are still comparable — a 24mm lens gives (approximately) the same field of view regardless of the mount distance of any given system. So, focal length does correspond to field of view.



                You note that using a macro adapter affects field of view. In fact, they also increase the focal length, although since macro extension tubes remove the ability to actually focus at infinity, we leave practical reality and get into the realm of theory.







                share|improve this answer














                share|improve this answer



                share|improve this answer








                edited 8 hours ago

























                answered 11 hours ago









                mattdm

                117k37343633




                117k37343633











                • Flange distance is very much independent of the effective focal length of the lens. one can design a lens to have any effective focal length one wishes, and then enclose said design in a mechanical casing which will result in any distance one wishs between the back principle plane of the lens and the sensor (which we can equate to the flange distance). the resulting magnification (and as an extension, the FOV) are derived from the above distance. this is basic geometrical optics.
                  – Yuval Weissler
                  11 hours ago






                • 2




                  In photography, lenses are marketed and sold as I have described. You are right that they are independent, but lenses are made and sold for a particular mount, and that means the flange focal distance of that mount is included in the number on the lens.
                  – mattdm
                  10 hours ago










                • This means that a 24mm lens on Pentax K mount will have approximately the same field of view as a 24mm lens for Canon or Nikon or whatever else, assuming the same sensor size.
                  – mattdm
                  10 hours ago










                • Your note that adding a macro adapter changes the focal length is not true - at least not from the physics point of view, which is what I'm interested in. The effective focal of any lens does not change, no matter how far from the sensor you choose to place it.
                  – Yuval Weissler
                  8 hours ago










                • @YuvalWeissler Again, the focal length of a complex lens is measured when that lens is focused at infinity. If you change the distance from the lens to the sensor, you will need to change the focus in order to make the focal plane coincide (that is "not have everything be all blurry"). This is true from "the physics point of view" and from any other point of view.
                  – mattdm
                  6 hours ago
















                • Flange distance is very much independent of the effective focal length of the lens. one can design a lens to have any effective focal length one wishes, and then enclose said design in a mechanical casing which will result in any distance one wishs between the back principle plane of the lens and the sensor (which we can equate to the flange distance). the resulting magnification (and as an extension, the FOV) are derived from the above distance. this is basic geometrical optics.
                  – Yuval Weissler
                  11 hours ago






                • 2




                  In photography, lenses are marketed and sold as I have described. You are right that they are independent, but lenses are made and sold for a particular mount, and that means the flange focal distance of that mount is included in the number on the lens.
                  – mattdm
                  10 hours ago










                • This means that a 24mm lens on Pentax K mount will have approximately the same field of view as a 24mm lens for Canon or Nikon or whatever else, assuming the same sensor size.
                  – mattdm
                  10 hours ago










                • Your note that adding a macro adapter changes the focal length is not true - at least not from the physics point of view, which is what I'm interested in. The effective focal of any lens does not change, no matter how far from the sensor you choose to place it.
                  – Yuval Weissler
                  8 hours ago










                • @YuvalWeissler Again, the focal length of a complex lens is measured when that lens is focused at infinity. If you change the distance from the lens to the sensor, you will need to change the focus in order to make the focal plane coincide (that is "not have everything be all blurry"). This is true from "the physics point of view" and from any other point of view.
                  – mattdm
                  6 hours ago















                Flange distance is very much independent of the effective focal length of the lens. one can design a lens to have any effective focal length one wishes, and then enclose said design in a mechanical casing which will result in any distance one wishs between the back principle plane of the lens and the sensor (which we can equate to the flange distance). the resulting magnification (and as an extension, the FOV) are derived from the above distance. this is basic geometrical optics.
                – Yuval Weissler
                11 hours ago




                Flange distance is very much independent of the effective focal length of the lens. one can design a lens to have any effective focal length one wishes, and then enclose said design in a mechanical casing which will result in any distance one wishs between the back principle plane of the lens and the sensor (which we can equate to the flange distance). the resulting magnification (and as an extension, the FOV) are derived from the above distance. this is basic geometrical optics.
                – Yuval Weissler
                11 hours ago




                2




                2




                In photography, lenses are marketed and sold as I have described. You are right that they are independent, but lenses are made and sold for a particular mount, and that means the flange focal distance of that mount is included in the number on the lens.
                – mattdm
                10 hours ago




                In photography, lenses are marketed and sold as I have described. You are right that they are independent, but lenses are made and sold for a particular mount, and that means the flange focal distance of that mount is included in the number on the lens.
                – mattdm
                10 hours ago












                This means that a 24mm lens on Pentax K mount will have approximately the same field of view as a 24mm lens for Canon or Nikon or whatever else, assuming the same sensor size.
                – mattdm
                10 hours ago




                This means that a 24mm lens on Pentax K mount will have approximately the same field of view as a 24mm lens for Canon or Nikon or whatever else, assuming the same sensor size.
                – mattdm
                10 hours ago












                Your note that adding a macro adapter changes the focal length is not true - at least not from the physics point of view, which is what I'm interested in. The effective focal of any lens does not change, no matter how far from the sensor you choose to place it.
                – Yuval Weissler
                8 hours ago




                Your note that adding a macro adapter changes the focal length is not true - at least not from the physics point of view, which is what I'm interested in. The effective focal of any lens does not change, no matter how far from the sensor you choose to place it.
                – Yuval Weissler
                8 hours ago












                @YuvalWeissler Again, the focal length of a complex lens is measured when that lens is focused at infinity. If you change the distance from the lens to the sensor, you will need to change the focus in order to make the focal plane coincide (that is "not have everything be all blurry"). This is true from "the physics point of view" and from any other point of view.
                – mattdm
                6 hours ago




                @YuvalWeissler Again, the focal length of a complex lens is measured when that lens is focused at infinity. If you change the distance from the lens to the sensor, you will need to change the focus in order to make the focal plane coincide (that is "not have everything be all blurry"). This is true from "the physics point of view" and from any other point of view.
                – mattdm
                6 hours ago












                up vote
                2
                down vote














                So my question is why do we characterize lenses with the focal length and not the effective field of view?




                The very good reason for cameras is that field of view also strongly depends on the sensor size. A small sensor captures a more narrow view than a larger sensor can.



                The lens focal length does provide some field of view, which the sensor size crops to capture possibly a lesser amount of it. Cameras with tiny sensors only capture a small field, so they have to use a much shorter focal length lens to compare to an expected "regular picture" view seen by other cameras with larger sensors. Crop Factor compares that sensor size view to the historical 35 mm film frame size view that so many of us are very familiar with.



                Focal length is NOT at all about the mounting flange distance. The internal focus node can be moved by design. A "telephoto" lens means that node is slightly in front of the front lens element (lens is shorter than the focal length). A retro-focus (wide angle) lens places that focus node behind the rear element, to create space behind the lens. But focal length is to that focal node.



                The focal length marked on the lens is when focused at infinity. Some imagine that is the definition of focal length, but it is merely one setting for it. Zoom lenses also vary focal length.



                When focused closer than infinity, the lens is extended forward (the frontal elements are, or possibly only internal elements) so that the internal focus node in the lens is further from the sensor plane. If we instead assume focal length is the distance from this internal node to the sensor plane, then of course the focal length is a bit longer when focused closer. That longer focal length changes things, like f-number, which can affect exposure, so regular lenses don't allow distances shorter than some nominal close distance, typically at around 0.1x magnification.






                share|improve this answer


























                  up vote
                  2
                  down vote














                  So my question is why do we characterize lenses with the focal length and not the effective field of view?




                  The very good reason for cameras is that field of view also strongly depends on the sensor size. A small sensor captures a more narrow view than a larger sensor can.



                  The lens focal length does provide some field of view, which the sensor size crops to capture possibly a lesser amount of it. Cameras with tiny sensors only capture a small field, so they have to use a much shorter focal length lens to compare to an expected "regular picture" view seen by other cameras with larger sensors. Crop Factor compares that sensor size view to the historical 35 mm film frame size view that so many of us are very familiar with.



                  Focal length is NOT at all about the mounting flange distance. The internal focus node can be moved by design. A "telephoto" lens means that node is slightly in front of the front lens element (lens is shorter than the focal length). A retro-focus (wide angle) lens places that focus node behind the rear element, to create space behind the lens. But focal length is to that focal node.



                  The focal length marked on the lens is when focused at infinity. Some imagine that is the definition of focal length, but it is merely one setting for it. Zoom lenses also vary focal length.



                  When focused closer than infinity, the lens is extended forward (the frontal elements are, or possibly only internal elements) so that the internal focus node in the lens is further from the sensor plane. If we instead assume focal length is the distance from this internal node to the sensor plane, then of course the focal length is a bit longer when focused closer. That longer focal length changes things, like f-number, which can affect exposure, so regular lenses don't allow distances shorter than some nominal close distance, typically at around 0.1x magnification.






                  share|improve this answer
























                    up vote
                    2
                    down vote










                    up vote
                    2
                    down vote










                    So my question is why do we characterize lenses with the focal length and not the effective field of view?




                    The very good reason for cameras is that field of view also strongly depends on the sensor size. A small sensor captures a more narrow view than a larger sensor can.



                    The lens focal length does provide some field of view, which the sensor size crops to capture possibly a lesser amount of it. Cameras with tiny sensors only capture a small field, so they have to use a much shorter focal length lens to compare to an expected "regular picture" view seen by other cameras with larger sensors. Crop Factor compares that sensor size view to the historical 35 mm film frame size view that so many of us are very familiar with.



                    Focal length is NOT at all about the mounting flange distance. The internal focus node can be moved by design. A "telephoto" lens means that node is slightly in front of the front lens element (lens is shorter than the focal length). A retro-focus (wide angle) lens places that focus node behind the rear element, to create space behind the lens. But focal length is to that focal node.



                    The focal length marked on the lens is when focused at infinity. Some imagine that is the definition of focal length, but it is merely one setting for it. Zoom lenses also vary focal length.



                    When focused closer than infinity, the lens is extended forward (the frontal elements are, or possibly only internal elements) so that the internal focus node in the lens is further from the sensor plane. If we instead assume focal length is the distance from this internal node to the sensor plane, then of course the focal length is a bit longer when focused closer. That longer focal length changes things, like f-number, which can affect exposure, so regular lenses don't allow distances shorter than some nominal close distance, typically at around 0.1x magnification.






                    share|improve this answer















                    So my question is why do we characterize lenses with the focal length and not the effective field of view?




                    The very good reason for cameras is that field of view also strongly depends on the sensor size. A small sensor captures a more narrow view than a larger sensor can.



                    The lens focal length does provide some field of view, which the sensor size crops to capture possibly a lesser amount of it. Cameras with tiny sensors only capture a small field, so they have to use a much shorter focal length lens to compare to an expected "regular picture" view seen by other cameras with larger sensors. Crop Factor compares that sensor size view to the historical 35 mm film frame size view that so many of us are very familiar with.



                    Focal length is NOT at all about the mounting flange distance. The internal focus node can be moved by design. A "telephoto" lens means that node is slightly in front of the front lens element (lens is shorter than the focal length). A retro-focus (wide angle) lens places that focus node behind the rear element, to create space behind the lens. But focal length is to that focal node.



                    The focal length marked on the lens is when focused at infinity. Some imagine that is the definition of focal length, but it is merely one setting for it. Zoom lenses also vary focal length.



                    When focused closer than infinity, the lens is extended forward (the frontal elements are, or possibly only internal elements) so that the internal focus node in the lens is further from the sensor plane. If we instead assume focal length is the distance from this internal node to the sensor plane, then of course the focal length is a bit longer when focused closer. That longer focal length changes things, like f-number, which can affect exposure, so regular lenses don't allow distances shorter than some nominal close distance, typically at around 0.1x magnification.







                    share|improve this answer














                    share|improve this answer



                    share|improve this answer








                    edited 2 hours ago

























                    answered 5 hours ago









                    WayneF

                    9,3711924




                    9,3711924




















                        up vote
                        1
                        down vote













                        Focal length and lens FOV are inherently the same thing. The variable is how much of the lens' FOV is utilized (i.e. extension tubes/TC's/crop sensor/etc). Why would you specify a lens' characteristic by something that may be variable?






                        share|improve this answer
























                          up vote
                          1
                          down vote













                          Focal length and lens FOV are inherently the same thing. The variable is how much of the lens' FOV is utilized (i.e. extension tubes/TC's/crop sensor/etc). Why would you specify a lens' characteristic by something that may be variable?






                          share|improve this answer






















                            up vote
                            1
                            down vote










                            up vote
                            1
                            down vote









                            Focal length and lens FOV are inherently the same thing. The variable is how much of the lens' FOV is utilized (i.e. extension tubes/TC's/crop sensor/etc). Why would you specify a lens' characteristic by something that may be variable?






                            share|improve this answer












                            Focal length and lens FOV are inherently the same thing. The variable is how much of the lens' FOV is utilized (i.e. extension tubes/TC's/crop sensor/etc). Why would you specify a lens' characteristic by something that may be variable?







                            share|improve this answer












                            share|improve this answer



                            share|improve this answer










                            answered 6 hours ago









                            Steven Kersting

                            1847




                            1847




















                                up vote
                                1
                                down vote













                                The focal length of a single lens always stays the same, the field of view changes as you focus on something closer than infinity, that may be a good reason to specify the focal length, and not the field of view.






                                share|improve this answer






















                                • The field of view changes because the actual focal length changes.
                                  – mattdm
                                  6 hours ago










                                • @mattdm Do you have any reference for that? According to thin lens equations the image distance also changes as the subject distance changes for a single simple lens element. Therefore the field of view changes too.
                                  – Orbit
                                  6 hours ago










                                • photo.stackexchange.com/questions/16549/…
                                  – mattdm
                                  5 hours ago






                                • 1




                                  This is not hard. Focal length is to where the lens focuses behind. The focal length marked on the lens is when focused at infinity. When focused at a closer distance, the lens elements must be moved forward to still focus on the same image plane. That can be several possible longer focal lengths (over a small range), so the one at infinity is simply the one marked. Any lens formula must use the actual focal length for the actual focused distance. If at 1:1 magnification, the focal length behind is the same as the focused distance in front. Or focused at infinity is half that behind then.
                                  – WayneF
                                  1 hour ago







                                • 1




                                  @Orbit We're talking about photographic lenses here, which are, excepting some novelty acts, complex lenses, not simple lenses, let alone an idealized thin lens.
                                  – mattdm
                                  44 mins ago














                                up vote
                                1
                                down vote













                                The focal length of a single lens always stays the same, the field of view changes as you focus on something closer than infinity, that may be a good reason to specify the focal length, and not the field of view.






                                share|improve this answer






















                                • The field of view changes because the actual focal length changes.
                                  – mattdm
                                  6 hours ago










                                • @mattdm Do you have any reference for that? According to thin lens equations the image distance also changes as the subject distance changes for a single simple lens element. Therefore the field of view changes too.
                                  – Orbit
                                  6 hours ago










                                • photo.stackexchange.com/questions/16549/…
                                  – mattdm
                                  5 hours ago






                                • 1




                                  This is not hard. Focal length is to where the lens focuses behind. The focal length marked on the lens is when focused at infinity. When focused at a closer distance, the lens elements must be moved forward to still focus on the same image plane. That can be several possible longer focal lengths (over a small range), so the one at infinity is simply the one marked. Any lens formula must use the actual focal length for the actual focused distance. If at 1:1 magnification, the focal length behind is the same as the focused distance in front. Or focused at infinity is half that behind then.
                                  – WayneF
                                  1 hour ago







                                • 1




                                  @Orbit We're talking about photographic lenses here, which are, excepting some novelty acts, complex lenses, not simple lenses, let alone an idealized thin lens.
                                  – mattdm
                                  44 mins ago












                                up vote
                                1
                                down vote










                                up vote
                                1
                                down vote









                                The focal length of a single lens always stays the same, the field of view changes as you focus on something closer than infinity, that may be a good reason to specify the focal length, and not the field of view.






                                share|improve this answer














                                The focal length of a single lens always stays the same, the field of view changes as you focus on something closer than infinity, that may be a good reason to specify the focal length, and not the field of view.







                                share|improve this answer














                                share|improve this answer



                                share|improve this answer








                                edited 29 mins ago

























                                answered 6 hours ago









                                Orbit

                                435312




                                435312











                                • The field of view changes because the actual focal length changes.
                                  – mattdm
                                  6 hours ago










                                • @mattdm Do you have any reference for that? According to thin lens equations the image distance also changes as the subject distance changes for a single simple lens element. Therefore the field of view changes too.
                                  – Orbit
                                  6 hours ago










                                • photo.stackexchange.com/questions/16549/…
                                  – mattdm
                                  5 hours ago






                                • 1




                                  This is not hard. Focal length is to where the lens focuses behind. The focal length marked on the lens is when focused at infinity. When focused at a closer distance, the lens elements must be moved forward to still focus on the same image plane. That can be several possible longer focal lengths (over a small range), so the one at infinity is simply the one marked. Any lens formula must use the actual focal length for the actual focused distance. If at 1:1 magnification, the focal length behind is the same as the focused distance in front. Or focused at infinity is half that behind then.
                                  – WayneF
                                  1 hour ago







                                • 1




                                  @Orbit We're talking about photographic lenses here, which are, excepting some novelty acts, complex lenses, not simple lenses, let alone an idealized thin lens.
                                  – mattdm
                                  44 mins ago
















                                • The field of view changes because the actual focal length changes.
                                  – mattdm
                                  6 hours ago










                                • @mattdm Do you have any reference for that? According to thin lens equations the image distance also changes as the subject distance changes for a single simple lens element. Therefore the field of view changes too.
                                  – Orbit
                                  6 hours ago










                                • photo.stackexchange.com/questions/16549/…
                                  – mattdm
                                  5 hours ago






                                • 1




                                  This is not hard. Focal length is to where the lens focuses behind. The focal length marked on the lens is when focused at infinity. When focused at a closer distance, the lens elements must be moved forward to still focus on the same image plane. That can be several possible longer focal lengths (over a small range), so the one at infinity is simply the one marked. Any lens formula must use the actual focal length for the actual focused distance. If at 1:1 magnification, the focal length behind is the same as the focused distance in front. Or focused at infinity is half that behind then.
                                  – WayneF
                                  1 hour ago







                                • 1




                                  @Orbit We're talking about photographic lenses here, which are, excepting some novelty acts, complex lenses, not simple lenses, let alone an idealized thin lens.
                                  – mattdm
                                  44 mins ago















                                The field of view changes because the actual focal length changes.
                                – mattdm
                                6 hours ago




                                The field of view changes because the actual focal length changes.
                                – mattdm
                                6 hours ago












                                @mattdm Do you have any reference for that? According to thin lens equations the image distance also changes as the subject distance changes for a single simple lens element. Therefore the field of view changes too.
                                – Orbit
                                6 hours ago




                                @mattdm Do you have any reference for that? According to thin lens equations the image distance also changes as the subject distance changes for a single simple lens element. Therefore the field of view changes too.
                                – Orbit
                                6 hours ago












                                photo.stackexchange.com/questions/16549/…
                                – mattdm
                                5 hours ago




                                photo.stackexchange.com/questions/16549/…
                                – mattdm
                                5 hours ago




                                1




                                1




                                This is not hard. Focal length is to where the lens focuses behind. The focal length marked on the lens is when focused at infinity. When focused at a closer distance, the lens elements must be moved forward to still focus on the same image plane. That can be several possible longer focal lengths (over a small range), so the one at infinity is simply the one marked. Any lens formula must use the actual focal length for the actual focused distance. If at 1:1 magnification, the focal length behind is the same as the focused distance in front. Or focused at infinity is half that behind then.
                                – WayneF
                                1 hour ago





                                This is not hard. Focal length is to where the lens focuses behind. The focal length marked on the lens is when focused at infinity. When focused at a closer distance, the lens elements must be moved forward to still focus on the same image plane. That can be several possible longer focal lengths (over a small range), so the one at infinity is simply the one marked. Any lens formula must use the actual focal length for the actual focused distance. If at 1:1 magnification, the focal length behind is the same as the focused distance in front. Or focused at infinity is half that behind then.
                                – WayneF
                                1 hour ago





                                1




                                1




                                @Orbit We're talking about photographic lenses here, which are, excepting some novelty acts, complex lenses, not simple lenses, let alone an idealized thin lens.
                                – mattdm
                                44 mins ago




                                @Orbit We're talking about photographic lenses here, which are, excepting some novelty acts, complex lenses, not simple lenses, let alone an idealized thin lens.
                                – mattdm
                                44 mins ago










                                up vote
                                0
                                down vote













                                I think we use focal length as attribute of lenses fro historical reasons. Time ago glass elements can be measured by distance from the focus (using Sun light) to the "center" of the element. Same is true for the lenses. To get field of view you need to do some mathematics and (maybe) this stop ordinary people (not mathematicians) to use it.



                                But this question IMHO is too broad and opinion based....






                                share|improve this answer
























                                  up vote
                                  0
                                  down vote













                                  I think we use focal length as attribute of lenses fro historical reasons. Time ago glass elements can be measured by distance from the focus (using Sun light) to the "center" of the element. Same is true for the lenses. To get field of view you need to do some mathematics and (maybe) this stop ordinary people (not mathematicians) to use it.



                                  But this question IMHO is too broad and opinion based....






                                  share|improve this answer






















                                    up vote
                                    0
                                    down vote










                                    up vote
                                    0
                                    down vote









                                    I think we use focal length as attribute of lenses fro historical reasons. Time ago glass elements can be measured by distance from the focus (using Sun light) to the "center" of the element. Same is true for the lenses. To get field of view you need to do some mathematics and (maybe) this stop ordinary people (not mathematicians) to use it.



                                    But this question IMHO is too broad and opinion based....






                                    share|improve this answer












                                    I think we use focal length as attribute of lenses fro historical reasons. Time ago glass elements can be measured by distance from the focus (using Sun light) to the "center" of the element. Same is true for the lenses. To get field of view you need to do some mathematics and (maybe) this stop ordinary people (not mathematicians) to use it.



                                    But this question IMHO is too broad and opinion based....







                                    share|improve this answer












                                    share|improve this answer



                                    share|improve this answer










                                    answered 11 hours ago









                                    Romeo Ninov

                                    2,89521123




                                    2,89521123



























                                         

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