Compression is Real

Started 11 months ago | Discussions thread
jrtrent Veteran Member • Posts: 4,983
Re: no cropping or stitching

Gerry Winterbourne wrote:

jrtrent wrote:

Gerry Winterbourne wrote:

jrtrent wrote:

The article the OP referred to starts out with, "We've been saying for years that the term "lens compression" is misleading," and perhaps that is true, but I prefer some of the articles they had in the more distant past. Below is an excerpt from one that was primarily discussing what a normal, or standard, lens is, and explains why I prefer using a normal focal length for most of my shots:

There's a saying "correlation isn't causation" and that applies to your quotation below.

"A telephoto lens, for example, renders distant objects larger in the frame,

True.

and has the effect of compressing the apparent spatial relationship between objects.

True. But these are two separate things that a telephoto lens does; it isn't the fact that things look larger that explains why perspective appears compressed.

A wide angle does precisely the opposite; in squeezing more content into the image, objects appear smaller and more distant.

This is more problematic - it suggests that everything in the frame looks more distant when, in fact, things in the foreground don't. What really happens is that the apparent difference in size of foreground and distant objects is what causes the apparent false perspective.

[Edit] And it's the fact that the telephoto lens cuts out the foreground objects (which would appear huge) that causes their apparent false perspective.

It's in the happy medium between these two extremes that the standard lens lies; the apparent sizes and spatial relationships between image elements appear natural, and much as they did in real life."

I have no problem with the quotation as a pragmatic guide to the visual effects of various lenses; but it shouldn't be taken as an explanation of those effects.

I can't disagree with anything you wrote. Regarding the problematic statement about the wide angle lens from Andy Westlake's article, I agree and think that Tamron's description covers the ground a little better.

I saw that post and thought the Tamron article had so much wrong with it that I'd just pass it by. But as you raise it again here I'll comment. I've pasted it here from your other post.

"A focal length approximating the diagonal dimension of the camera’s image plane will render an angle-of-view with negligible magnification—similar to normal human vision.

Magnification is the ratio of the size of the image on the sensor to the actual size of the subject; 1:1 is full size, the usual definition of what macro lenses give. Macro lenses come in all focal lengths; relating magnification to FL is nonsense - but see * below.

The full angle of view of human vision is over 180 degrees horizontally and about half that vertically. The horizontal and vertical AOV of a lens whose diagonal equals FL are about 45 and 30 degrees - a fraction of the human AOV.

Focal lengths numerically lower than normal will render negative magnification,

* This shows that the terminology is garbled. What it seems to be saying is that whatever the actual magnification of a normal lens is, a longer FL will give more than that while a shorter FL will give less.

resulting in wider angles-of-view (wide angle), while those numerically greater than normal render positive magnification, producing narrower angles-of-view (telephoto.)."

So while the above is true it has nothing to do with perspective.

"A photographic lens provides a visual effect, making closely located subjects larger while remotely located subjects smaller.

This is back to front. Things nearer the viewer look bigger than equal things further away. That's a natural visual effect and is what we call perspective. Lenses don't provide that effect, they merely project it onto the film or sensor.

As the focal length becomes shorter in a wideangle lens, this perspective difference expands making closely located subjects even bigger and remotely located ones even smaller (exaggerated perspective).

This is fundamentally wrong, as explained many times in this thread. The focal length itself has no effect whatsoever on perspective. The only thing that affects perspective is the viewpoint. Different FLs offer different AOVs and that allows the photographer to change the viewpoint. But it's the viewpoint that controls perspective, not the FL that allows the photographer to choose the viewpoint.

In contrast, in a telephoto lens, as focal lengths become longer, less difference is observed between close and distant subjects, making it appear as if they are closer regardless of the distance between them (compressed perspective)."

Ditto.

"Generally speaking, a focal length range that provides a similar perspective to the human eye is considered to be somewhere between 40-60mm."

It isn't so considered by anyone who understands what perspective is. As long as the camera is just in front of the eye than every FL provides the same perspective as that eye.

Using lenses of different FLs can give photographs whose apparent perspective differs from the actual perspective from that viewpoint; and longer FLs often yield apparent compression of perspective while shorter FLs often yield apparent exaggeration of perspective. I'm not disputing this as a fact.

I appreciate your detailed post, but it will come as no surprise to you (given our past exchanges) that I'm not really understanding what you're getting at. If the preceding paragraph is true, then I don't see how it can also be true that "The focal length itself has no effect whatsoever on perspective" (from your paragraph just above "Ditto"). I appreciate your stressing the word apparent, but I thought that was what Tamron was saying, too, in talking about lenses providing a "visual effect" and how long lenses "make it appear as if" distant things are nearer.

What I am saying is that the article you quote is completely wrong as an explanation of the fact. Anyone who understands the true explanation can see what the article is trying to get at; it's just unfortunate that it bungles the attempt.

Perhaps some day I will understand the true explanation. Next chance I get, I'll spend some happy hours at the library reading every photography book they've got that covers this subject. I've done that before but don't recall finding anything substantially different from Tamron's explanation. I could just be misremembering, or possibly I'll see something different when re-reading these books and looking more closely for differences. As it stands, I'm at least content to know that my use of normal focal length lenses gives my pictures the apparent perspective that I prefer.

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