This post will probably upset a few of you. Don’t care. Lens compression is a myth, I’ve been saying it for years, but when I try to explain why, peoples’ eyes start to glaze over. This video from Dave McKeegan, however, explains and demonstrates the principle wonderfully. Although, as Dave says, it really doesn’t matter.
Dave does go very in-depth into explaining the technical side of why lens compression doesn’t really exist, and if you’re not technically minded in the least, you’ll probably want to watch some parts of the video two or three times to fully understand what the demonstrations… uh, demonstrate. But it’s worth sticking with it. The better you know the principles, the better you’ll be able to use your gear.
But in short, “compression” is a combination of perspective, angle of view and depth of field. None of which really have anything to do with the specific lens you’re using. Yeah, that sounds counterintuitive, but bear with it.
Perspective is the position at which your camera resides to take the shot relative to the scene before it. If you have two lenses of wildly different focal lengths, like 35mm and 150mm shooting from the same position, both images will provide the same perspective. If you set the aperture on both lenses to an equal diameter, they’ll also have the same depth of field. And if you crop that 35mm image down to the angle of view of the 150mm lens, and scale them to match, both images will effectively be identical, from an optical standpoint.
Notice I said “equal diameter” on the aperture, and not an equal f/stop number. It’s not the f/stop that needs to be the same between the two lenses of different focal lengths to get the same depth of field – this we know – but the same physically sized iris opening. To get the same depth of field between these two lenses, the 35mm in Dave’s demonstration was at f/1.4. The 150mm lens was at f/8. Dave explains why that happens, too, and where we start to hit the limits of physics and current lens technology.
To bypass current lens technology limitations, though, you can see this explanation from 2018 using 3D software that effectively has infinite resolution. It uses the Dolly Zoom to explain the same point – and why “zooming with your feet” is also nonsense.
As I said, it’s a pretty in-depth video, but if you’re into geeking out and understanding the underlying principles, or if you just want to understand why certain lenses behave in a certain way, it’s well worth a watch.