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Focal Length and Perspective.
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Apr 5, 2021 07:20:52   #
R.G. Loc: Scotland
 
Part of the problem with this subject is the undefined use of the term "perspective". In a purely geometric sense it can be shown that if your position doesn't change, the relative placement of the various objects in your field of view - and your perception of them - doesn't change. It's a simple fact of geometry. Focal length is not a factor.

What focal length does affect is the field of view. As the focal length of a lens increases it has a magnifying effect on what you're seeing (due to the field of view becoming narrower). That magnification affects the perception of distance - including relative distance (and therefore depth) - and the resulting compression effect is often described as a change of perspective - but it's just a narrowing of the area of our attention. That can be proved easily by taking a shot that was captured using a wide angle lens and cropping it to give the same framing as a shot taken with a telephoto lens from the same spot. Apart from a difference in resolution (due to the cropping), they will be exactly the same.

Where our eyes are concerned, you can think of them as behaving like a very wide angle lens with a very small sweet spot. Within the sweet spot the focus, detail perception (resolution), contrast and colour perception are all good, and the further out from the sweet spot you go the worse they all become. At the periphery of our vision our eyes are still capable of detecting movement, large objects and areas of high contrast but almost nothing in the way of fine detail, and provide us with very little colour information.

Where human perception is concerned, another factor that comes into play is our attention. We can change the focus of our attention by changing the direction of our gaze, and in addition to that we can also focus our attention at will within any given field of view (a fixed field of view is what a fixed gaze gives us). The sweet spot is large enough to give us the visual information (the detail, colour, contrast, depth perception etc) that we need when we want to focus our attention on specific areas of interest.

In the case of photographs we want the freedom to choose where within the frame the areas of interest occur, so cameras have to be capable of providing a high level of visual information throughout the frame. Put another way, the sweet spot needs to cover the entire frame (the entire field of view). In reality the very edges of the frame aren't as important as the centre, so where lens design is concerned, the edges of the field of view aren't as critical as the centre, but lenses still get criticised if the edges are soft in any way.

PERSPECTIVE.

So what about the perspective provided by our eyes? To reiterate, focal length affects the field of view but it doesn't affect perspective. What does affect perspective is position. Provided your position doesn't change, the perspective that you get using a short focal length (a wide angle lens) is the same as what you get with longer focal lengths.

One implication of those points is that the human eye doesn't have a unique perspective based on its focal length. The only thing that changes the perspective that we see with our eyes is our position - which changes every time we move. If you want a camera to give you the same perspective that your eyes do, you need only to stand in the same position to take the photograph - focal length is irrelevant.

PERCEPTION.

The sweet spot of our eyes (as described above) corresponds roughly to the area of interest that we get when we focus our attention, which apparently is covered by an angle of view of roughly 40° - 60°. For a full frame (FF) camera, that corresponds roughly to the angle of view provided by a lens with a focal length of ~43mm. So in terms of human perception, a 45mm FF lens will cover the area that corresponds (approximately) to the area of our focused interest, and that in turn corresponds (approximately) to the sweet spot of human eyes (which is only a small part of the wider field of view provided by our eyes).

Put another way, a 45mm FF lens gives us a condensation of what we see with our eyes by isolating the area that we normally focus on and excluding the wider context provided by our peripheral vision.

However, excluding context may not be the photographer's intention, and if his/her intention is to focus the viewer's attention, that can be achieved by other means, the main one being framing. In that context, focal length will often be irrelevant. In addition to that, lenses are usually designed to provide a sweet spot that covers the entire field of view, and that in turn allows areas of interest to be placed anywhere within the frame. In that context, focal length is irrelevant.

Where the perception of distance (and depth) within a photograph is concerned, much depends on how the photograph is viewed. The main relevant factors are the size of the display (the monitor, print or whatever) and the viewing distance. And where the photograph itself is concerned, the degree of cropping is a factor. All of these factors are independent of focal length.

It can be seen from these points that a 45mm (FF equivalent) focal length has very limited significance in photography. However, one of the main things that focal length does do is convey a sense of what the photographer's viewpoint was at the time of capture. The photographer's viewpoint is shared with the viewer, and in that context, the choice of focal length determines the starting point. Bearing in mind that the angle of view can be changed by cropping, we can say that in very general terms a wide angle of view (a short focal length) will give the viewer the impression that they are distanced from the captured scene (or the action), and as the angle of view becomes narrower (as the focal length increases) the viewer is given the impression that they are being brought closer into the scene or closer to the action.

In that context a 45mm FF lens will be good at conveying the sense that the viewer is seeing what they would be seeing if they were standing at a realistic distance from the scene or the action. The relevance of that point will depend very much on circumstance.

Apr 5, 2021 08:11:32   #
Leitz Loc: Solms
 
This is basic stuff that any amateur ought to know. You could have written it all out in one line!

Apr 5, 2021 08:14:43   #
larryepage
 
R.G. wrote:
Part of the problem with this subject is the undefined use of the term "perspective". In a purely geometric sense it can be shown that if your position doesn't change, the relative placement of the various objects in your field of view - and your perception of them - doesn't change. It's a simple fact of geometry. Focal length is not a factor.

What focal length does affect is the field of view. As the focal length of a lens increases it has a magnifying effect on what you're seeing (due to the field of view becoming narrower). That magnification affects the perception of distance - including relative distance (and therefore depth) - and the resulting compression effect is often described as a change of perspective - but it's just a narrowing of the area of our attention. That can be proved easily by taking a shot that was captured using a wide angle lens and cropping it to give the same framing as a shot taken with a telephoto lens from the same spot. Apart from a difference in resolution (due to the cropping), they will be exactly the same.

Where our eyes are concerned, you can think of them as behaving like a very wide angle lens with a very small sweet spot. Within the sweet spot the focus, detail perception (resolution), contrast and colour perception are all good, and the further out from the sweet spot you go the worse they all become. At the periphery of our vision our eyes are still capable of detecting movement, large objects and areas of high contrast but almost nothing in the way of fine detail, and provide us with very little colour information.

Where human perception is concerned, another factor that comes into play is our attention. We can change the focus of our attention by changing the direction of our gaze, and in addition to that we can also focus our attention at will within any given field of view (a fixed field of view is what a fixed gaze gives us). The sweet spot is large enough to give us the visual information (the detail, colour, contrast, depth perception etc) that we need when we want to focus our attention on specific areas of interest.

In the case of photographs we want the freedom to choose where within the frame the areas of interest occur, so cameras have to be capable of providing a high level of visual information throughout the frame. Put another way, the sweet spot needs to cover the entire frame (the entire field of view). In reality the very edges of the frame aren't as important as the centre, so where lens design is concerned, the edges of the field of view aren't as critical as the centre, but lenses still get criticised if the edges are soft in any way.

PERSPECTIVE.

So what about the perspective provided by our eyes? To reiterate, focal length affects the field of view but it doesn't affect perspective. What does affect perspective is position. Provided your position doesn't change, the perspective that you get using a short focal length (a wide angle lens) is the same as what you get with longer focal lengths.

One implication of those points is that the human eye doesn't have a unique perspective based on its focal length. The only thing that changes the perspective that we see with our eyes is our position - which changes every time we move. If you want a camera to give you the same perspective that your eyes do, you need only to stand in the same position to take the photograph - focal length is irrelevant.

PERCEPTION.

The sweet spot of our eyes (as described above) corresponds roughly to the area of interest that we get when we focus our attention, which apparently is covered by an angle of view of roughly 40° - 60°. For a full frame (FF) camera, that corresponds roughly to the angle of view provided by a lens with a focal length of ~43mm. So in terms of human perception, a 45mm FF lens will cover the area that corresponds (approximately) to the area of our focused interest, and that in turn corresponds (approximately) to the sweet spot of human eyes (which is only a small part of the wider field of view provided by our eyes).

Put another way, a 45mm FF lens gives us a condensation of what we see with our eyes by isolating the area that we normally focus on and excluding the wider context provided by our peripheral vision.

However, excluding context may not be the photographer's intention, and if his/her intention is to focus the viewer's attention, that can be achieved by other means, the main one being framing. In that context, focal length will often be irrelevant. In addition to that, lenses are usually designed to provide a sweet spot that covers the entire field of view, and that in turn allows areas of interest to be placed anywhere within the frame. In that context, focal length is irrelevant.

Where the perception of distance (and depth) within a photograph is concerned, much depends on how the photograph is viewed. The main relevant factors are the size of the display (the monitor, print or whatever) and the viewing distance. And where the photograph itself is concerned, the degree of cropping is a factor. All of these factors are independent of focal length.

It can be seen from these points that a 45mm (FF equivalent) focal length has very limited significance in photography. However, one of the main things that focal length does do is convey a sense of what the photographer's viewpoint was at the time of capture. The photographer's viewpoint is shared with the viewer, and in that context, the choice of focal length determines the starting point. In very general terms a short focal length will give the viewer the impression that they are distanced from the captured scene (or the action), and as the focal length increases the viewer is given the impression that they are being brought closer into the scene or closer to the action.

In that context a 45mm FF lens will be good at conveying the sense that the viewer is seeing what they would be seeing if they were standing at a realistic distance from the scene or the action. The relevance of that point will depend very much on circumstance.
Part of the problem with this subject is the undef... (show quote)


Perspective is a very difficult topic to discuss without having a common scene to visualize together. Fine shades of meaning can also cloud the discussion. I will agree that as long as nothing moves, point of view does not change. In the three-dimensional world we live in, that means that the content of the height/width plane doesn't change with focal length. (Although the "extent," or how much we can see of it, does change, sometimes drastically.) But the depth field does change, also sometimes drastically.

The best way to really see this is to go outside with a pair of binoculars. Look at a scene. Study it carefully. Now use your hands to crop the scene. Nothing has moved. You just see less of it. Now look at the same area through the binoculars. The extent is also reduced, but something else has happened. Things have moved closer together in one direction (the distance direction) while moving farther apart in the other directions (height and width). This second change is unimportant, because the same elements are visible and the same elements are hidden behind other elements as before. Note that walking closer to the subject would likely have revealed some previously hidden elements and might have hidden others. It would have changed the perspective.

Now turn the binoculars around and look through them from the objective end. The effect us opposite, and the relative "relocation" of elements will appear even more dramatically, even though what is visible and what is not will again not change.

If you have a hard time seeing this, take the binoculars to an upper floor of a building and repeat the experiment by looking out a window. Do the binoculars change the height of the building? I'll leave it for you to work that out.

Perspective is a complex subject. But I have learned over the years that simply changing lenses is not the same (at least not exactly the same) as moving closer to or farther from your subject. In fact, that is exactly why we prefer 85-100mm lenses over 50mm lenses (for full frame, anyway) for portraiture.

 
 
Apr 5, 2021 08:20:07   #
larryepage
 
Leitz wrote:
This is basic stuff that any amateur ought to know. You could have written it all out in one line!


Actually, I have found that it is not. That is why so many folks don't get it right. Or why so many folks want an extreme wide angle lens for landscapes until they figure out that things are more complicated than just putting a short focal length lens on a camera.

Apr 5, 2021 08:30:07   #
R.G. Loc: Scotland
 
larryepage wrote:
.....Things have moved closer together in one direction (the distance direction) while moving farther apart in the other directions (height and width).....


That is what a lot of people are mistakenly referring to as a change in perspective. If the only thing my opening post achieves is a clarification of what "perspective" means it will have achieved something.

Apr 5, 2021 08:51:12   #
Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)
 
R.G. wrote:
Part of the problem with this subject is the undefined use of the term "perspective". In a purely geometric sense it can be shown that if your position doesn't change, the relative placement of the various objects in your field of view - and your perception of them - doesn't change. It's a simple fact of geometry. Focal length is not a factor.

What focal length does affect is the field of view. As the focal length of a lens increases it has a magnifying effect on what you're seeing (due to the field of view becoming narrower). That magnification affects the perception of distance - including relative distance (and therefore depth) - and the resulting compression effect is often described as a change of perspective - but it's just a narrowing of the area of our attention. That can be proved easily by taking a shot that was captured using a wide angle lens and cropping it to give the same framing as a shot taken with a telephoto lens from the same spot. Apart from a difference in resolution (due to the cropping), they will be exactly the same.

Where our eyes are concerned, you can think of them as behaving like a very wide angle lens with a very small sweet spot. Within the sweet spot the focus, detail perception (resolution), contrast and colour perception are all good, and the further out from the sweet spot you go the worse they all become. At the periphery of our vision our eyes are still capable of detecting movement, large objects and areas of high contrast but almost nothing in the way of fine detail, and provide us with very little colour information.

Where human perception is concerned, another factor that comes into play is our attention. We can change the focus of our attention by changing the direction of our gaze, and in addition to that we can also focus our attention at will within any given field of view (a fixed field of view is what a fixed gaze gives us). The sweet spot is large enough to give us the visual information (the detail, colour, contrast, depth perception etc) that we need when we want to focus our attention on specific areas of interest.

In the case of photographs we want the freedom to choose where within the frame the areas of interest occur, so cameras have to be capable of providing a high level of visual information throughout the frame. Put another way, the sweet spot needs to cover the entire frame (the entire field of view). In reality the very edges of the frame aren't as important as the centre, so where lens design is concerned, the edges of the field of view aren't as critical as the centre, but lenses still get criticised if the edges are soft in any way.

PERSPECTIVE.

So what about the perspective provided by our eyes? To reiterate, focal length affects the field of view but it doesn't affect perspective. What does affect perspective is position. Provided your position doesn't change, the perspective that you get using a short focal length (a wide angle lens) is the same as what you get with longer focal lengths.

One implication of those points is that the human eye doesn't have a unique perspective based on its focal length. The only thing that changes the perspective that we see with our eyes is our position - which changes every time we move. If you want a camera to give you the same perspective that your eyes do, you need only to stand in the same position to take the photograph - focal length is irrelevant.

PERCEPTION.

The sweet spot of our eyes (as described above) corresponds roughly to the area of interest that we get when we focus our attention, which apparently is covered by an angle of view of roughly 40° - 60°. For a full frame (FF) camera, that corresponds roughly to the angle of view provided by a lens with a focal length of ~43mm. So in terms of human perception, a 45mm FF lens will cover the area that corresponds (approximately) to the area of our focused interest, and that in turn corresponds (approximately) to the sweet spot of human eyes (which is only a small part of the wider field of view provided by our eyes).

Put another way, a 45mm FF lens gives us a condensation of what we see with our eyes by isolating the area that we normally focus on and excluding the wider context provided by our peripheral vision.

However, excluding context may not be the photographer's intention, and if his/her intention is to focus the viewer's attention, that can be achieved by other means, the main one being framing. In that context, focal length will often be irrelevant. In addition to that, lenses are usually designed to provide a sweet spot that covers the entire field of view, and that in turn allows areas of interest to be placed anywhere within the frame. In that context, focal length is irrelevant.

Where the perception of distance (and depth) within a photograph is concerned, much depends on how the photograph is viewed. The main relevant factors are the size of the display (the monitor, print or whatever) and the viewing distance. And where the photograph itself is concerned, the degree of cropping is a factor. All of these factors are independent of focal length.

It can be seen from these points that a 45mm (FF equivalent) focal length has very limited significance in photography. However, one of the main things that focal length does do is convey a sense of what the photographer's viewpoint was at the time of capture. The photographer's viewpoint is shared with the viewer, and in that context, the choice of focal length determines the starting point. Bearing in mind that the angle of view can be changed by cropping, we can say that in very general terms a wide angle of view (a short focal length) will give the viewer the impression that they are distanced from the captured scene (or the action), and as the angle of view becomes narrower (as the focal length increases) the viewer is given the impression that they are being brought closer into the scene or closer to the action.

In that context a 45mm FF lens will be good at conveying the sense that the viewer is seeing what they would be seeing if they were standing at a realistic distance from the scene or the action. The relevance of that point will depend very much on circumstance.
Part of the problem with this subject is the undef... (show quote)


I agree with everything you've written. But I do have to add one observation. When using extremely wide angle lenses, extension distortion is a real perspective problem. When objects are close to the camera they are magnified to be "bigger than life" and as the distance increases, things start to go back to a more "normal" perception. This phenomenon of "forced perspective" was explored creatively in the old days, using forward tilt on view and technical cameras and it wasn't nearly as exaggerated as it is when you use a 10mm lens on a crop camera, or a 14mm on a full frame camera. Nearly everyone who gets such a lens goes through a period of discovery during the initial "honeymoon" but many quickly grow tired of that "look" and the lenses become infrequently used. When every landscape photograph has a giant cluster of rocks or flowers dominating the foreground while the beautiful waterfall or mountain range in the distance occupies less than 10% of the frame, it gets old pretty quickly.

Apr 5, 2021 09:56:09   #
Thomas902 Loc: Washington DC
 
Wide angle "distortion" is oft used in marketing campaigns.
Yields a larger than life "perspective" to showcase products.
And it doesn't take extreme wide angle glass, only how effectively it is used...

Even Nikon is culpable here...


(Download)

 
 
Apr 5, 2021 10:00:48   #
R.G. Loc: Scotland
 
Gene51 wrote:
I agree with everything you've written. But I do have to add one observation. When using extremely wide angle lenses, extension distortion is a real perspective problem. When objects are close to the camera they are magnified to be "bigger than life" and as the distance increases, things start to go back to a more "normal" perception. This phenomenon of "forced perspective" was explored creatively in the old days, using forward tilt on view and technical cameras and it wasn't nearly as exaggerated as it is when you use a 10mm lens on a crop camera, or a 14mm on a full frame camera. Nearly everyone who gets such a lens goes through a period of discovery during the initial "honeymoon" but many quickly grow tired of that "look" and the lenses become infrequently used. When every landscape photograph has a giant cluster of rocks or flowers dominating the foreground while the beautiful waterfall or mountain range in the distance occupies less than 10% of the frame, it gets old pretty quickly.
I agree with everything you've written. But I do h... (show quote)


Thanks for your input. Having fun with distorted faces is probably another source of amusement that could be mentioned .

There's a definite cautionary aspect to your comments. Spending money on gear that ends up not being used is a subject that deserves its own thread.

Apr 5, 2021 10:04:23   #
R.G. Loc: Scotland
 
Thomas902 wrote:
Wide angle "distortion" is oft used in marketing campaigns.
Yields a larger than life "perspective" to showcase products.
And it doesn't take extreme wide angle glass, only how effectively it is used...

Even Nikon is culpable here...


Good example. If they wanted to impress me they'd use photos that diminish their cameras' size, not exaggerate it .

Apr 5, 2021 17:24:01   #
Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)
 
R.G. wrote:
Thanks for your input. Having fun with distorted faces is probably another source of amusement that could be mentioned .

There's a definite cautionary aspect to your comments. Spending money on gear that ends up not being used is a subject that deserves its own thread.


Ah the infamous "horse face" . . . a perfect example of extension distortion run amok

Apr 5, 2021 18:33:18   #
Leitz Loc: Solms
 
larryepage wrote:
Actually, I have found that it is not. That is why so many folks don't get it right. Or why so many folks want an extreme wide angle lens for landscapes until they figure out that things are more complicated than just putting a short focal length lens on a camera.

True, but the effect of various focal lengths is very clear in the viewfinder. Hard to not understand it.

 
 
Apr 5, 2021 19:12:46   #
larryepage
 
Leitz wrote:
True, but the effect of various focal lengths is very clear in the viewfinder. Hard to not understand it.


It's hard to understand a lot of things that folks don't understand about photography.

Apr 5, 2021 20:07:50   #
Leitz Loc: Solms
 
larryepage wrote:
It's hard to understand a lot of things that folks don't understand about photography.



Apr 6, 2021 05:57:37   #
R.G. Loc: Scotland
 
Leitz wrote:
True, but the effect of various focal lengths is very clear in the viewfinder. Hard to not understand it.


If it was all so easy to understand there wouldn't be so much misunderstanding surrounding subjects like perspective and the use of that word. Some of the effects of focal length are obvious but it's not the obvious stuff that causes misunderstanding.

Apr 6, 2021 07:37:22   #
Leitz Loc: Solms
 
R.G. wrote:
If it was all so easy to understand there wouldn't be so much misunderstanding surrounding subjects like perspective and the use of that word. Some of the effects of focal length are obvious but it's not the obvious stuff that causes misunderstanding.

Over the years I have observed more explanations than questions about perspective. However, I've had little to do with inexperienced photographers, which may be why I've not seen much misunderstanding.

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