Lens and perspective distortion: they’re not the same thing

What does lens distortion mean? It might sound obvious, but it’s easy to confuse distortion caused by the lens, such as barrel distorion with wide-angle lenses, with the perspective distortion caused by tilting the camera upwards to capture tall buildings. It’s important to treat these as two separate things.

Lens distortion and how to fix it

Almost all zoom lenses show some kind of distortion, and quite a few prime lenses too. It’s one of the compromises in lens design, and its often very difficult to fix distortion completely without expensive optical designs or without compromising on some other area of the lens’s performance or features.

Lens distortion is where straight lines in the real world show up as slightly bent or bowed lines in the photo. You notice this particularly with buildings and horizon lines. The most common kind of distortion is barrel distortion, where straight lines near the edge of the frame appear to bow outwards. This is very common, almost inevitable, with wide-angle lenses. With some it’s hardly visible but with others it’s really obvious.

There’s another less common kind of distortion that you sometimes see with telephoto lenses, and especially with long-zoom lenses at or near their maximum focal length. Called pincushion distortion, it’s the opposite of barrel distortion. With pincushion distortion, straight lines near the edge of the frame appear to bow inwards, as if the edges of the picturee are being pinched inwards.

Before the mirrorless era, lens makers tried quite hard to correct distortion, but it was still necessary sometimes to use lens correction profiles in software, or for the maker to offer in-camera distortion correction.

Now, it’s common for makers to design lenses that depend on digital corrections. Distortion is pretty easy to correct in software and it’s a way for them to concentrate on other aspects of lens design. Some makers correct in-camera JPEGs automatically and embed correction profiles in RAW files so that editing software can load and apply them. Others rely on software companies to supply correction profiles of their own, and this is common practice too. 

One way or another, you’re unlikely to be left without proper lens corrections whether you shoot JPEGs or RAW files. In many instances you may never actually see an uncorrected version of your images.

Perspective distortion and how it happens

Perspective distortion is where straight lines which should be parallel appear to converge instead. A very common real-world example is where tall buildings appear to narrow towards the top. This is sometimes called keystoning, or converging verticals. You can get horizontal convergence too, or a mixture of both.

You’re much more likely to see this kind of perspective distortion with wide-angle lenses, which leads some people to blame wideangle lenses for ‘distorting’ the subject. In fact it’s not the lens’s fault, it’s the photographers.

It’s because of the way we use wide-angle lenses and why we choose them in the first place. Typically, you choose a wide-angle lens to get more into the scene or because you can’t quite move far enough away from your subject to get it all in.

So far so good. But if you’re standing at street level but you’re photographing a tall building, inevitably you’re going to tilt the camera upwards, and it’s this that’s causing the perspective distortion. If you keep the camera level, it won’t happen. It’s not always possible, of course, but if you can move just a little further away to do this, and maybe find some foreground subject to make the composition work, there will be no perspective convergence.

Alternatively, you can use a perspective control or ‘shift’ lens. These are heavy, expensive and slow to use, but they fix the problem in a different way. They let you shift the lens upwards relative to the camera so that the camera stays level but the framing changes to include tall buildings.

Another approach is to correct any perspective distortion digitally. This does work, but the stronger the convergence, the more you’re going to notice a drop in quality in the most heavily corrected areas.

Always fix lens distortion before correcting perspective distortion

This is important. Most software will apply lens corrections automatically, but do check this has been done before you attempt any perspective corrections. This is because it becomes much harder to judge whether straight lines are properly horizontal or vertical if there is still some element of bowing or distortion in these straight lines.

Correcting perspective distortion by eye is much easier once any lens distortion has been removed. And if your software can correct vertical and horizontal convergence vertically, it’s going to do a much better job if lens corrections have already been applied.