depth "compression" with long FL lenses

[donS]

I'm interested in the apparent "compression" of depth that occurs with long focal length lenses.  Cannot find existing discussions of this topic in the Bob Atkins forum.  Am I missing a relevant posting?  If not, can anyone supply some conventional wisdom on this topic?

Many thanks

don

[KeithB]

While it doesn't address it directly, this test shows it in action.

http://www.luminous-landscape.com/tutorials/dof2.shtml

I believe it is simply the change of perspective.

[Patibo]

Hello DonS,
KeithB was talking about depth of field, but I think by "depth compression" you meant the change in perspective that occurs when objects move further away. This not only a property of camera lenses, it's reality in general. For example, think about a street pavement that is made of tiles 1ft by 1 ft. When you look at a tile that is at a distance of 1ft from you the nearest edge of it is 1ft away from you, the further edge 2ft. That's the double, so you will see a strong perspective, that means the closer part of the tile looks big and the further part looks small, with a large relative difference. Now look at a tile 10ft away. The nearest edge is 10ft away, the furthest 11ft. That is only 10% difference, so you will see a weak perspective, meaning the nearest part of the tile looks only a little bit bigger that the furthest part.
A standard lens, a lens that has a focal length equal to the diagonal of your sensor or film, will give you the same perspective as your eyes. A wide angle lens allows you to capture objects in your frame that are closer without appearing bigger, so the perspective will appear exaggerated. A telephoto lens will allow you to fill the frame with an object that is far away, thus making it appear bigger. Therefore the perspective will look compressed. In fact, an object that is close by will have the same perspective with a telephoto lens as with a wide angle lens but you will not see this because you will only see a narrow part of the object. Also, if you take a picture of an object far away with a wide angle lens and you enlarge the object so that it looks the same size as it does with the telephoto lens, the perspective will be the same for both lenses!

[KeithB]

Pablo, while the article was about depth of field, it clearly shows the compression.

However, by coincidence, I came across this article:
http://www.lightandmatter.org/2011/general-photography-articles/learn-photography/composition/photo-composition-lens-choice/

which discusses it directly.

[Bob Atkins]

For an explanation you can think it in terms of magnification.

Say you have subjects of the same size which are 10m apart. Say you need to stand 100m away with a 1000mm lens to get the closer subject to fill the frame. Since magnification is given (approximately) by focal length/distance, the magnification for the closer subject is 1/100 while for the second subject it's 1/110. Those numbers are pretty close (only about 10% different) so the two subjects appear to be about the same size and that makes them seem close(r) together.

Now with a 10mm lens you'd need to shoot from a distance of 10m to get the closer subject the same size. The magnification would still be 1/100. However the magnification of the more distant subject would now be 1/200. The difference in magnification is now much larger, in fact it's a factor of two, so the more distant subject now appears much further away because it's much smaller.

The difference in size of the two subjects is what contributes to (and gives rise to) the level of perceived  'compression".