The Ansel Adams Fallacy: "True perspective depends only on the camera-to-subject distance"

Hi Tom

You've written quite extensively about this issue and I admit I still don't understand the significance of the point you are making. For me, the myth about perspective in practical photography is ensconced in the phrase "telephoto compression". You see this phrase everywhere - the implication being if you want to photograph something so the final image looks like a collage of overlapping layers with no depth between the layers, you use a tele lens because that is what causes that effect. Yet it is quite easy to demonstrate exactly the same "compression" effect with a wide lens if you crop a tiny section from the horizon and blow it up big. It is the camera to subject distance that creates the differences in foreground/background subject sizes that photographers like to play with rather than the lens focal length, yet the message you see everywhere is that it is focal length that creates the effects. Darwin Wiggett has a nice little easy to understand demo using video: petapixel.com/2016/06/09/use-zoom-lens-tell-story/ It shows very nicely how you use focal length to control the apparent size of the background and your feet to control the apparent size of the foreground (and thus the size relationship between foreground and background i.e. perspective).

It seems to me that you are suggesting you can achieve these affects simply by changing your viewing position relative to the print. It would be interesting if you could generate all these perspective effects (Big Nose Effect, Layered Collage Effect) just by changing the viewing distance from the print.

Is that what you are saying you can do, or am I simply misunderstanding the 10k words you must have written on the subject by now (more than possible!)?

Cheers

Dave

Precisely! It is the enlargement of the image that causes the telephoto compression (provided you view it from the same distance). If you take any image and enlarge it enough it will show telephoto compression, provided the subject matter is suitable. It is just the same as looking through a telescope. An 8x telescope (or binoculars) enlarges the image you see by 8 times, which makes everything appear 8 times closer, i.e. 1/8th the distance.

Of course, instead of enlarging the image, you can simply move closer to it or even use a magnifying glass to view it very closely. Viewing an image from a shorter distance from the image is exactly the same as enlarging the image and viewing it from the same distance.

Usually our photographs contain too little detail to be able to enlarge them 10 times or more. The result is just too blurry to be acceptable. However, if you look at some of the gigapixel images available online, you can zoom in to 10 times enlargement and still see a good amount of detail. If you zoom right in, telephoto compression is usually apparent, while if you zoom right out, wide-angle distortion occurs (if the image uses rectilinear projection).

Changing the camera position changes the relative sizes of two objects at different distances from the camera. Their absolute size in the image is dependent on the focal length.

Changing the viewing distance changes the absolute size (i.e. angular size seen by the eye) of everything in the picture. That automatically changes our judgement of distance:

object size / object distance = image size / image distance

Yes, perspective distortion effects are caused by viewing distance from the print, except for the big nose effect, which is not the same sort of effect. You can see it for yourself without a camera. Simply put your eye three inches from someone's nose. It is not perspective distortion (according to the usual definition) and is not caused by the viewing distance (from the photograph).

Wide-angle perspective distortion is caused by viewing distance (between viewer and photograph), but the big nose effect is caused by camera distance (between camera and nose).

Viewing distance is always relative to the focal length (scaled by the enlargement factor). If the viewing distance is equal to the focal length (scaled by the enlargement factor), then the image is seen with correct perspective, i.e. exactly the same perspective seen from the camera position. For greater viewing distances wide-angle perspective distortion occurs, for smaller viewing distances telephoto compression occurs.

I am sorry, but you have misunderstood what I am trying to explain.

I think part of the problem is that we are so used to looking at photographs and other images that we see them primarily as 2-D images and don't try to imagine that we are looking at a real scene.

Consider this scenario: You are watching someone walking down the road away from you. That person's image in your eye gets smaller and smaller as he gets further and further away. You naturally interpret the fact that the image gets smaller as an indication that he is moving further away.

However if you have a photograph and look at it while you move away from it, the image of the photograph in your eye gets smaller and smaller and you interpret that as the photograph getting further and further away rather than the objects in the photo getting further away.

Instead, if you imagine that you are looking at a real scene rather than just a photograph, then as the image gets smaller and smaller, your brain will interpret this as everything in the image getting further and further away. If the photo moves twice as far from your eye, then everything in the photo appears twice as far away. This means that an object that appeared to be 10m away now appears to be 20m away and an object that appeared to be 20m away now appears to be 40m away. So, the distance between those two objects appears to have increased from 10m to 20m. That is the expansion of perspective that appears when an image becomes smaller.

Telephoto compression occurs if you move closer to the photo so that everything in it appears closer, all by the same factor. Distances from the viewer all appear to shrink.

I hope this helps,
Cheers,
Tom.

These discussions inevitably get wordy and hard to parse. I'm still not sure whether we are talking about the same thing. I've tried to dream up a scenario that explains what I mean by the word "perspective" (and how it works):

1. Imagine a tall man holding a 2m ruler standing in front of your camera (say 5m away). The man and the rule are the same height.

2. You take a picture with a standard lens. You examine the picture and conclude that the ruler is 2m tall and the man is 2m tall.

3. You increase your shooting distance to 10m, then 20m.

4. You examine the images and you note that as the shooting distance increases, both the man and the ruler diminish in size in the frame equally. At 20m they take up a lot less of the frame than at 5m but both are the same relative size as each other in all the shots.

The shrinking of the image size with distance is, I think, what most people think of as normal and what they mean by perspective: the further away something is, the smaller it looks.

1. Next, you repeat the whole exercise, except you shoot one sequence of shots with a mild tele lens and another with a mild wide angle.

2. You examine the resulting pictures and you notice the same pattern: the closer the shooting distance, the larger the figures in the image, the further the shooting distance, the smaller the figures. You also notice that the tele lens renders the figures larger at all shooting distances than the wide lens; but that in all shots the man and the ruler remain the same relative sizes. The man is still the same height as the ruler no matter what shooting distance is used and no matter what focal length is employed. Their absolute size varies with shooting distance and focal length but the relative height to the pair remains the same under all conditions.

3. Next we do something different: the man drives the ruler into the ground so it is self supporting. He then walks backwards so the ruler is fixed at 5m and he is at 10m. You examine the images. As you would expect, the man is rendered smaller than when he stands next to the ruler. This time the relative height of the man and ruler is different. This effect holds for all shooting differences and all focal lengths - even though the absolute height of both will vary as distance and focal length change, the fact that the man and the ruler no longer look the same size is the key factor.

This is what I mean by perspective in photography and why I say that shooting distance is the only factor that can make a change to the relative heights of the man and the ruler. I really struggle to imagine that viewing distance from the print can possibly influence the relative heights at all.

Telephoto compression

1. This time we complicate the situation slightly. We add a second man to the frame.

2. We repeat the exercise of shooting the ruler and both men standing side by side at 5m and we see the exact same result in an image: They all look the same size.

3. Then we wildly change things. This time we leave the ruler stuck in the ground at 5m but the first man stands 100m away and the second man stands 130m away. We then examine the print. What do we see?
   i) A large foreground ruler
   ii) Two tiny men in the background
   iii) This is the crucial thing: the two men look almost the same size despite being quite far apart! Why is this! The answer is because the 30m distance between them is relatively small compared to the 130m distance to the camera. There is a slight size difference between the two men, but we don't really notice it in the print, they look a similar size. Instant telephoto compression - except you don't need a tele lens to achieve it, it happens at all focal lenghths

The reason, I believe, that so many people attribute this effect to the tele lens rather than the relative distance between camera and the two men is because a tele lens allows you render things that are far away big enough in the frame so that you notice the apparent compression. The second reason is the reduced angle of view cuts out subjects at closer distances removing anything that distracts you from the visual illusion of "telephoto compression". But it's not focal length caused at all.

As to your story of this being something that is attributed to viewer distance from the print, I'm afraid I still don't understand this at all.

Ok... but this has little to do with addressing the common misapprehension repeated all over photography literature that compression effects are the result of using a telephoto lens rather than relative camera to subject distances...

I can't say that I have ever noticed any distortion from changing viewing distance. It must be a subtle effect. I will look for it now.

It's actually much simpler than you think!

Take a photo of a person standing 10 metres away. Make a print of the photo. Stand at the camera position and hold that print in front of you while that same person is standing 10m from you in their original location.

Find the distance at which you have to hold the print for the image of the person to be exactly the same size as your view of that person in reality. I am talking about the angular size as seen by your eye. That is the viewing distance needed to give correct perspective, i.e. the same perspective as you see with your own eyes when standing at the camera position.

Now move the print closer: the person appears closer to you, as does everything else in the print. That's telephoto compression of the perspective.

Move the print further away: the person appears further away, as does everything else in the print. That's wide-angle expansion of the perspective.

It's your last two sentences that confuse me. I don't use the word "perspective", "expansion" and "compression" to describe what you say. To me, you can only use those terms if something within the frame changes size relative to something else in the frame. If everything gets bigger or smaller, there's no compression going on.

From a photographic composition point of view, what we are trying to do is change the relative size of subject elements to create a certain look eg a gigantic dog sitting in front of a tiny house or a tiny dog dwarfed by a gigantic house

Er, I can no longer see anything sharply that is closer than about 4 feet (120cm) away. I need longer arms these days. Reading glasses are my friend.

Perspective distortion does not cause a change in the relative sizes of things in the image. It causes the absolute size of everything in the image to change by the same factor.

If everything doubles in size, then everything appears to be only half as far away (compression).

If everything halves in size, then everything appears to be twice as far away (expansion).

This is not the same as moving the camera position.

So this debate is not about physics but about terminology. Fair enough,happy with that. I must say the rather than "perspective distortion" as the term to describe this, I would just stick with "perspective".

I'll investigate whether viewing distance makes any obvious difference to me. So far, I have looked through my Finn Hopson book "Fieldwork" at arms' length and again at nose to the page distance (of course everything is blurry) and I don't see anything obvious changing to the geometry, but I'll take your word for it and keep trying. Maybe it takes a while to build up awareness of what you are seeing.

I got to say, though, that even assuming you are 100% correct, it seems to me to be a storm in a teacup and have little practical relevance to the art of photography (at least compared to the effects from changing camera-to-subject differences). That kind of perspective change can be effectively used for compositional effect. Your concepts seem more like correcting geometric distortion from pointing the camera up or down slightly. Yes, to some people, achieving geometric straightness is important, but not to a lot of people. It's hardly something that would bother me for 1 second, to be frank, and I can't imagine any way that it can be used for composition. As someone else already said, viewing distance is not something the photographer has any control over.

Thanks for spending the time and effort to explain it, though.

It is about physics, but I have tried to avoid the mathematics.

If you are having trouble seeing the difference in perspective as you change the size of an image (or change viewing distance, both have the same effect), try this:

When you are next driving your car, compare what you see in the different rear-view mirrors. The mirror on the windscreen gives a life-size view and most drivers find that it is easy to estimate the distance of the vehicle behind when looking in this mirror.

On the other hand, the side mirrors usually give a wide-angle view in which the image is reduced in size and distances look greater than they are in reality. When using the side mirrors, it is easy to over-estimate the distance to the vehicle behind you.

Do you agree?

I agree. You can be an excellent photographer without understanding any of this. Yet many online tutorials (some of which I gave links to earlier in this thread) have been created at least partly to promote the Ansel Adams Fallacy (of course, they don't call it that)!

I think it is important that when incorrect science is being promoted, it should be called out for what it is - incorrect.

[deleted]

It is a fallacy to claim that perspective is determined solely by camera position.

Perspective depends on both the camera position (relative to the scene) and the viewer's position (relative to the image). This is explained clearly in the Manual of Photography by Jacobson et. al. (7th edition, 1978), and other places. It is incorrect science when people claiming to explain perspective distortion fail to mention the significance of the viewer's position relative to the image.

If you claim that perspective depends only on camera position, then the onus is on you to prove it.