Compression in photography refers to how the subject and the background appear to be compressed together onto the same plane when using a telephoto lens. The effect increases the longer the focal length of the lens is.
An example of compression
In the photo below shot at 250mm, you’ll notice the effect of compression which is that that the objects in the background are closer in scale to the subject. In other words, it appears the objects are on a plane closer to where the subject is. Literally the perspective in the image is compressed towards the plane of focus. This also applies in reverse to anything in the foreground.
What causes compression?
Compression is basically perspective. When you use a long lens like a 500mm you have a very narrow field of view so all the light rays entering the lens are highly collimated. Because of that the effects of perspective are reduced. If the light rays entering the lens were perfectly collimated there would be no perspective at all and objects in the foreground or miles away in the background would all be the same scale.
The fact that we see compression more with a long focal length does not make compression unique to long lenses. To simulate the compression of a longer lens all you have to do is crop the image from a wider lens. To do this, stand at the same distance from the subject you would for a longer focal length then simply crop your image to the intended subject, you’ll find that the amount of compression is exactly the same. However there will be less background blur because the background will not be magnified as much with a shorter focal length. This is why crop sensors used with full frame lenses appear to increase the focal length of the lens.
To illustrate how cropping affects compression have a look at these two images. The first one is shot at 200mm.
The next image is cropped down to about 516mm equivalent framing.
Even cropped down by 2.6x on a 26MP sensor the image is still 4mp which is good enough for most web viewing.
How do photographers use compression?
Compression makes images more dramatic. The background can seem to tower above the subject in a way that is exciting and interesting to the viewer. It’s different from how we see with our eyes which naturally have a wider angle of view. We also rely on the effect of perspective to help us understand where things are in space. So you could say compression makes the image kind of surreal as well.
Best lenses for compression
Compression is best noticed with longer telephotos, which as you may know can be very expensive and heavy. But to get a strong effect of compression you really only need to get to 200mm. So a nice 70-200mm should fit the bill 90% of the time. For even more compression that you can still easily carry around with you try a zoom in the range of 100-400mm.
Of course a 300mm f2.8 or a 400mm f2.8 will give you lots of compression for those seriously dramatic shots plus the option of completely blurring out the background.
I think compression should be explored more in photography and I plan on actively seeking out opportunities to use more compression in the future.
Let us know what you think about using compression in the comments below.