Understanding and Mastering Lens Compression

This interview initially appeared on at Photograph IO, a blog dedicated to learning the ins and outs of photography. You can read it here.

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One of the most misunderstood concepts in photography is the compression effect resulting from various focal lengths. Understanding the mechanics behind it and how it works is an essential thing to do if you are already understand the basics behind exposure, composition and wish to furthermore improve your photography.

First of all, before explaining how to use “lens compression” in various photography cases, let’s define what is and what isn’t focal length compression.

1) There is no actual compression involved

This one might confuse a lot of you, and it surely has confused me for a long time. All lenses have some minor optical flaws, such as barrel distortion, pincushion distortion, etc. But all lenses, except for fisheyes, are rectilinear in design and thus render a straight line in real life a straight line in the picture. The sole factor determining how compressed your image will look like is your distance to your subject and nothing more. Let me explain this for you with two examples :

However, for the sake of simplicity, I’ll just call it lens compression for the rest of the article even if we now know that is has nothing to do with the lens.

2) Think of zooming as cropping to make things easier

One of the practical effects of lens compression is seen when using a zoom lens. When you zoom from the wide end to the tele end of your lens, you are basically reducing the FOV (field of view) of your lens. Assuming that neither you or the subject moves while you are zooming, there is absolutely NO perspective difference between the wide angle and telephoto views. And changing your FOV is, perspective wise, exactly the same thing as cropping/uncropping a picture. To put it in other words, zooming is exactly like cropping your picture (without losing any resolution). But where in the world does lens compression has anything to do with this?

Of course, real life situations are quite different from the theoretical explanation. Although you may stay close to a subject using a wide-angle, a telephoto lens usually forces you to stand back further away from the subject if you want to keep it about the same size. And changing the distance between you and your subject, of course, means a change of perspective. And that is why it is recommended to always shoot portraits at 50mm, 85mm or longer.

3) Crop factors makes it confusing

We usually talk of focal lengths in absolute terms (a 18mm APS-C lens is a 18mm lens). However, we have to use relative focal lengths when it comes to measuring the lens compression. A 200mm lens mounted on an APS-C camera (1.5x crop factor) lens will give the same perspective and amount of lens compression as a 300mm mounted on a full-frame body, since both have the same equivalent focal length. A 25mm MFT lens (2x crop factor) will give the same perspective and amount of lens compression as a 50mm full-frame lens. The logic behind that is simple : since zooming/de-zooming is equivalent to cropping/un-cropping, than a lens that is twice as cropped (50mm vs 25mm), but on a sensor twice as long (36mm vs 18mm), will give the exact same perspective for any shot.

Using focal length compression to your advantage

Apart from not shooting portraits with wide-angle lenses unless you want to distort your subject on purpose, there isn’t really any rule on what focal length to use in most cases. The greater perspective difference seen in wider-angle shots usually makes a picture more appealing due to its feeling of ‘depth’. As a general rule, unless bokeh is an important part of the shot, it is best to shoot from the widest angle possible without distorting your subject. Don’t spam the 200mm or 300mm for portraits if a 50mm f/1.8 can do it better. Using your feet is always your first and best option, and long/super telephotos are really meant for the rare cases where you just can’t get closer to your subject.

All rules are meant to be broken, however. As you get better and better at mastering the different focal lengths to manipulate the depth in a picture, don’t hesitate to use it creatively. Even though landscapes are often shot using wide-angle lenses, some of the best ones are shot using telephoto lenses. Or take wildlife for example. To the extent you can do it, it can really pay off if you can snap those animals using a normal or short tele lens instead of a super-tele. By shooting more and thinking consciously on the impact your choice of focal length makes to your pictures you will be eventually able to instinctively know which focal length to use for each situation.

A special note for ultra-wide lenses

Usually, people tend to get closer using wider-angle lenses and further away using telephoto lenses. However, a lot of people buy ultra-wide lenses as a way to ‘fit more’ in a single frame without having to stitch pictures to widen a shot (aka they stand back really far from the subject). While this is certainly one way to use these lenses, the real power of the ultra-wide is when you use it to get super-close, not farther away, from your subject. For instance, if you are shooting a landscape scene, don’t back off and try to fit the whole frame. Instead, get real close to the ground and exaggerate the size of the grass compared to the rest of the shot!

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Originally published at www.photographio.com on July 14, 2014.