However good your lens may be, you need to know how to spot (and correct) lens distortion. In this post we will explain everything photographers need to know to get the best results from their lenses. We’ll start by answering some of the most common questions about lens distortion, then look at several examples of lens distortion and, finally, try to cut through some of the technical jargon and put some of these concepts into layman’s terms.
Are you suggesting my pricey lens isn’t perfect?
All lenses have optical defects, and create images that aren’t perfect copies of the subjects they’re pointed at. However hard manufacturers try to create flawless optics, there’s simply no way to manufacture a lens that doesn’t suffer from distortions and aberrations to some degree.
So if I pay more money for my lens, I will get a less distorted picture from it?
Cost isn’t necessarily a quality indicator. The amount of distortion your lens suffers is largely dependent on the type of lens and its construction. Price plays a role, but factors such as the focal length are just as important.
The wider angled the lens, the harder it is for straight lines not to appear curved, for instance. Zooms are more prone to distortion than primes – simply because it’s impossible to correct for aberrations at every focal length.
This is no guarantee that a prime lens is perfect – but it’s true that the greater the zoom range (such as with superzooms) the more noticeable these distortions become.
I’ve never noticed any problems with my lens…
That may well be true for many of us. The thing is that lens design has improved tremendously in recent years, and the unforgiving precision of the latest digital sensors has forced the progress in lens design to accelerate. With a good digital SLR and a reputable lens, these distortions are subtle – but they are there.
We’ve never had it so good, then?
Definitely! But something as basic as darkening at the corner of the image is still a problem with modern pictures, just as it was for the Victorian photography pioneers. It’s just that this vignetting effect is not quite so marked these days. In fact, we tend to accept that pictures will be slightly darker at the edges – so we don’t even notice it (and some make their corners darker in Photoshop to exaggerate the effect). Take a picture of an evenly lit white surface, look at it carefully on your computer, and you’ll soon see if it’s brighter in the middle. This darkening effect can be eliminated using custom settings on certain cameras, or by using standard image-editing software.
How many different types of optical distortion are there?
There are dozens of these defects, from astigmatism right the way through to coma, but there are two or three that are particularly worth paying attention to.
Start me off with the easiest to understand, then…
We’ll kick off with curvilinear distortion. This comes in several different types, but the one you’ll see most commonly is barrel distortion. This is really easy to spot when you use an ultra-wide lens, and causes straight lines at the edge of the frame to bow outwards. The effect is even more obvious on a fisheye lens, where these distortions are left uncorrected by the designers in order to get the widest possible field of view.
What other curvilinear distortions are there?
Pincushion distortion is often seen on long telephoto lenses – and causes lines to bend inwards. The effect is usually subtle, and isn’t normally noticeable unless you’re photographing rectangular subjects straight on. Some zooms can show signs of moustache distortion – where one image can show both pincushion and barrel distortion. It’s most commonly seen with wide-angle zooms, and causes straight lines to appear wavy.
What else should I watch out for?
Chromatic aberration is probably the biggest bugbear of the modern SLR photographer. As we zoom in to our pictures, the tell-tale colour fringing that this causes is far more noticeable than back in the days of film (when it would only be seen in big blow-ups).
Where am I most likely to see chromatic aberration?
It affects lenses of all focal lengths, but will be more pronounced on extreme focal lengths, and with less expensive lenses. It’s also worth looking at lens tests for this phenomenon because it affects some models more than others. You’ll see it at the edges of subjects, and towards the edges of the image. It’s easiest to see where you have a white line crossing a darker area – window frames in your picture are a good place to look.
Can I do anything about it?
Yes, it can be fixed during editing. Your camera may even come with a program that will help you correct the problem. Photoshop CS has some good tools for minimising its effect on your pictures. Elements 8 users are not so fortunate, but separate distortion-correcting utilities are available. PTLens is well-regarded, can be tried for free and costs just $25.