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Understanding Lens Distortions

by Steph Doran (follow)
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Image taken by Adrian Pink

Have you ever looked at photo you’ve taken and thought it looked slightly distorted? A building seems crooked or a face looks really elongated or flat, and you can’t figure out why it looks that way?

This is the effect of lens distortion, when the optics of a lens cause straight lines within an image to appear bent or curved (referred to as an aberration). Because of this tendency to produce ‘curved’ lines, such distortion is also sometimes referred to as ‘curvilinear.’

Distortion comes in many forms, and sometimes several types of distortion can occur at the same time. Lens manufacturers try to counteract distortion in their lenses as much as possible, but some lenses are more prone to distortion than others.

Image taken by David Summers

Which Lenses Show Distortion?

Distortion is usually most obvious in wide angle lenses. Because we use wide angle lenses to capture very broad scenes, they naturally have to have a little distortion to ‘fit in’ such an expansive view.

Sometimes this type of distortion can be intentional. Most lenses are designed to ‘match’ the perspective of the human eye (rectilinear), but some curvilinear lenses, such as fish-eye lenses, are designed to warp this perspective. For example, a photograph of a diagonal road will appear straight throughout the entire image on a rectilinear lens, but will appear warped or curved on a fish-eye lens. However warping is very distracting in an image when it isn’t deliberate, like in the photo below.

Zoom lenses are also more prone to distortion than primes, because of the many different focal lengths they have. The higher the range of focal lengths within a lens, the more likely you are to notice distortion. The distortion you would get at the lowest focal length in a zoom lens will be different to what it is at the highest focal length, like in the photos below.

Image taken by Lindsay Adler

This is not to say that prime lenses are perfect - but it is easier for manufacturers to correct an aberration if the focal length is fixed, rather than variable.

Types Of Lens Distortion

Barrel: This type of distortion causes vertical and horizontal lines to bow outwards towards the edge of the frame. This causes rectangles and squares to appear ‘barrel shaped,’ hence the name. This type of distortion is often found in wide angle lenses because the lens’ field of view is wider than the camera’s sensor, so it needs to be squashed in to fit.

Pincushion: Pincushion distortion causes vertical and horizontal lines to pinch inwards towards the centre of the frame. It is the opposite of barrel distortion. This type of distortion is commonly seen in long or telephoto lenses. It occurs because the lens’ field of view is narrower than the camera’s sensor, and so it needs to be stretched to fit (the opposite of barrel distortion.)

Moustache/Complex: This type of distortion is a combination of barrel and pincushion distortion, and is the most difficult to deal with. Moustache distortion both pinches and bulges an image, but in different areas. This gives straight lines a ‘wavy’ appearance. For this reason, it is very difficult to fix in post production. Moustache distortion usually occurs in zoom lenses.

Chromatic Aberration: Chromatic aberration is visible as highly saturated lines around the edges of objects in your image. Almost like a double image, which doesn’t quite seem to line up correctly. For this reason, chromatic aberration is also known as ‘colour fringing’. It occurs when a lens fails to bring the wavelengths of every colour to the same focal plane, and is usually most visible around the edges of an image. There are three colour wavelength combinations to watch out for: purple with green, red with cyan, and blue with yellow.

Vignetting: A vignette is a gradual darkness around the edges of an image. Vignetting is often used as an editing technique to draw the eye towards the centre of the frame, but when it occurs unintentionally, it is seen as a lens defect.

How To Correct Lens Distortions

Image taken by Brooke Tasovac

Most lens distortions can be easily fixed with RAW processing software (such as Adobe Camera Raw or Lightroom). These programs allow you to adjust pin-cushion and barrel distortion, as well as vertical and horizontal shifts. To make things even easier, these programs often have a ‘lens profiles’ menu, for all different lens makes and models. By selecting the right one, automatic adjustments are made specifically to fix the distortions of that particular lens, which saves a lot of time. If you can’t remember the lens you used, check your metadata - the focal length will be listed for each individual image.

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