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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.
focal-length field-of-view
edited 3 hours ago
scottbb
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asked 12 hours ago
Yuval Weissler
1142
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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.
focal-length field-of-view
edited 3 hours ago
scottbb
18.2k75388
asked 12 hours ago
Yuval Weissler
1142
add a comment |Â
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.
focal-length field-of-view
edited 3 hours ago
scottbb
18.2k75388
asked 12 hours ago
Yuval Weissler
1142
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
focal-length field-of-view
edited 3 hours ago
scottbb
18.2k75388
asked 12 hours ago
Yuval Weissler
1142
edited 3 hours ago
scottbb
18.2k75388
asked 12 hours ago
Yuval Weissler
1142
edited 3 hours ago
scottbb
18.2k75388
edited 3 hours ago
scottbb
18.2k75388
edited 3 hours ago
scottbb
18.2k75388
18.2k75388
asked 12 hours ago
Yuval Weissler
1142
asked 12 hours ago
Yuval Weissler
1142
asked 12 hours ago
Yuval Weissler
1142
1142
add a comment |Â
add a comment |Â
5 Answers
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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.
answered 11 hours ago
mattdm
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
 |Â
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up vote
2
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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.
answered 5 hours ago
WayneF
9,3711924
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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?
answered 6 hours ago
Steven Kersting
1847
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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.
answered 6 hours ago
Orbit
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
 |Â
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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....
answered 11 hours ago
Romeo Ninov
2,89521123
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5 Answers
5
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.
answered 11 hours ago
mattdm
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
 |Â
show 2 more comments
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.
answered 11 hours ago
mattdm
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
 |Â
show 2 more comments
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.
answered 11 hours ago
mattdm
117k37343633
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.
answered 11 hours ago
mattdm
117k37343633
edited 8 hours ago
answered 11 hours ago
mattdm
117k37343633
answered 11 hours ago
mattdm
117k37343633
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
 |Â
show 2 more comments
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
 |Â
show 2 more comments
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.
answered 5 hours ago
WayneF
9,3711924
add a comment |Â
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.
answered 5 hours ago
WayneF
9,3711924
add a comment |Â
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.
answered 5 hours ago
WayneF
9,3711924
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.
answered 5 hours ago
WayneF
9,3711924
edited 2 hours ago
answered 5 hours ago
WayneF
9,3711924
answered 5 hours ago
WayneF
9,3711924
answered 5 hours ago
WayneF
9,3711924
9,3711924
add a comment |Â
add a comment |Â
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?
answered 6 hours ago
Steven Kersting
1847
add a comment |Â
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?
answered 6 hours ago
Steven Kersting
1847
add a comment |Â
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?
answered 6 hours ago
Steven Kersting
1847
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?
answered 6 hours ago
Steven Kersting
1847
answered 6 hours ago
Steven Kersting
1847
answered 6 hours ago
Steven Kersting
1847
answered 6 hours ago
Steven Kersting
1847
1847
add a comment |Â
add a comment |Â
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.
answered 6 hours ago
Orbit
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
 |Â
show 5 more comments
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.
answered 6 hours ago
Orbit
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
 |Â
show 5 more comments
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.
answered 6 hours ago
Orbit
435312
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.
answered 6 hours ago
Orbit
435312
edited 29 mins ago
answered 6 hours ago
Orbit
435312
answered 6 hours ago
Orbit
435312
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
 |Â
show 5 more comments
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
 |Â
show 5 more comments
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....
answered 11 hours ago
Romeo Ninov
2,89521123
add a comment |Â
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....
answered 11 hours ago
Romeo Ninov
2,89521123
add a comment |Â
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....
answered 11 hours ago
Romeo Ninov
2,89521123
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....
answered 11 hours ago
Romeo Ninov
2,89521123
answered 11 hours ago
Romeo Ninov
2,89521123
answered 11 hours ago
Romeo Ninov
2,89521123
answered 11 hours ago
Romeo Ninov
2,89521123
2,89521123
add a comment |Â
add a comment |Â
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