Telephoto Lens and "Foreshortening"... Etc...

[ameerat42]

From a discussion in another thread I tried a small experiment to test the idea of
"Foreshortening" or perhaps "Compressed Perspective" or perhaps other such
expressions when shooting with telephoto lenses.

My assertion is that "perspective" changes ONLY when you change the subject distance
AND NOT when you change only the focal length of the lens.

By "perspective" I mean the relative placements of different subjects in the field of view.
This means "distances" and "angles" between them, as far as can be determined in an image.

Below are two shots of a scene taken from the same position - subject distance - and with
only the focal length changed between the two shots - 93mm and 244mm respectively.
You can check the EXIF.

This is not an exhaustive test - I could have used 50mm as well, but...

If you put one image over the other and scale one so that they match in subject size,
then they will show exactly the same scene.

The ~2.6X difference in the FLs used shows NO relative "foreshortening" or "compression" of
perspective of the common field of view. Ie, It DOESN'T happen!

F=93mm view...
SDIM8634-lr.jpg

F=244mm view...
SDIM8635lr.jpg

[Tannin]

My assertion is that "perspective" changes ONLY when you change the subject distance
AND NOT when you change only the focal length of the lens.

Just so.

There is no need to assert it, AM. At least no more need than there is to assert that a stone rolls downhill or that the included angles of a triangle sum to 180.

Naturally, the truth of your observation (I won't call it an "assertion" as it's a basic truth of geometry) only becomes obvious to the casual eye when the two shots from different distances are constrained to the same framing (by cropping, or changing either the focal length or the sensor size). This is presumably why people talk about "the perspective of a given focal length" where what they actually mean is "the perspective of a given camera to subject distance". Well ... that's what they actually mean if they know their fundamental geometry, which is as basic to photography as achieving focus or exposure, and much more easily mastered. It is surprising how many people don't. As evidence, I proffer my new sig, which despite having been live for some weeks has apparently gone straight through to the keeper.

Mind you, it is often convenient to say "the perspective of a 300mm lens" as shorthand for "the perspective provided by a relatively large distance from the subject while retaining reasonably tight framing", and sometimes clearer too - provided, of course, that both speaker and listener understand that it is just a handy shorthand and not strictly true. In the same way, we say things like "my dog loves sticking his head out of the car window because he likes the feel of the wind in his face" where what he is actually enjoying is the feel of his rapidly-traveling face in the still air.

(As an aside, I often think that the benefit of flattening perspective by using a long focal length (or, if you prefer, by standing further away from the subject than one otherwise might) is greatly under-rated, especially by landscape and architectural photographers, and possibly even portrait shooters. That is one of the attractive things about a lot of sport and wildlife photography to my mind: one is often forced to shoot from a good distance away and the perspective is more natural. Another benefit (particularly useful for sport) is that framing changes more slowly at a greater distance. If you want, for example, to take pictures of a cyclist coming towards you, you might have time to take ten shots at 500mm between the time she fills a reasonable amount of the frame and the time she comes too close to fit in the picture, where at 50mm you you might only get one. The same applies with birds in flight.)

[ameerat42]

Ta, Tony. Of course, "assert/ion" is in the formal argument sense - something that has not been proven/illustratied/demonstrated/finished arm-waving about at
the time it is stated. - But I know you know that...

Good to talk photography, isn't it