What Is Lens Distortion And How To Avoid It
You may think that once you have bought a decent lens for your SLR, or a compact camera with a decent lens built-in, you can just forget about it.
Unfortunately, there is no such thing as a perfect lens, just as it's hard to find a perfect person, and there will always be some distortion – you just notice it a lot more with cheaper lenses. It's not necessarily about spending more, however. Certain types of lenses, such as wide-angles and zooms, will always be prone to some distortion.
One of the easiest ways to spot lens distortion is to attach a wide-angle lens, or shoot at the wide end of a zoom lens. The wider angled the lens, the more curved that straight lines will appear, as you can see with the distorted edges in the image below, taken with an otherwise good-quality, 24-70mm f/2.8 zoom.
A prime lens with a similar focal length, e.g. 24mm, will reveal less distortion as it's easier for makers to compensate for distortion when they only have one focal length to worry about. This relative lack of distortion is one of the reason that prime 85mm lenses are considered the 'perfect' portrait lens, for example.
Again, this is easily spotted when using a more extreme wide-angle lens – you'll notice straight lines at the edges of the picture 'bowing out,' like the shape of a barrel. It's obvious on a fisheye lens, too. Indeed barrel distortion helps to give fisheye lenses their distinctive look.
This effect, were lines do the opposite and bend inwards, is a common problem with longer telephoto lenses. Sometimes you have to look hard to spot this effect, but it's pretty obvious if you are shooting a rectangular shape, such as a window frame.
Apart from distortion to the shape of objects and lines and edges, lens distortion can cause fringing. Use a cheap and nasty lens and zoom in on some edges and you can often spot chromatic aberration, or colour fringing. It tends to be less of a problem on better quality lenses, and is also harder to spot at more conservative focal lengths.
How to minimise lens distortion
As mentioned, you shouldn't get OCD about lens distortion and try to eradicate it altogether. It's a fact of life, but having a good range of lenses (not just the cheap 18-5mm kit lens that came with your camera) will mean it's less of a headache.
Prime lenses tend to reveal less distortion, and you can pick up some bargain 50mm and 85mm examples online – they tend to have constant wide apertures too, making them great for people shots. It's also got a lot easier to fix lens distortion in software.
Lightroom's Lens Corrections tool is great. Use the Basic tab for simple correction of chromatic aberration (the program should be able to recognise your lens from its database) or go to the Manual option for a finer degree of control over vertical and horizontal distortion, scale, and other parameters.
Just be careful, however, as it's easy to widen or lengthen people at the same time as you are straightening the edges of buildings behind them!