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What is Distortion?

The Basics:

Distortion is a term used to define a series of aberrations [See Aberration] that occur with certain optical choices (such as type of lens) and shooting perspectives.

A Little More Detail:

There are two types of distortion: optical and perspective. Each contain various aberrations. Let’s break them down.
Optical Distortion
This is when distortions are created in an image based on lens type, including:
  • Barrel distortion - when straight lines in an image start to bend outward from the centre, creating a barrel-like effect. This can generally be seen in wide-angle prime lenses and occurs because the field of view is wider than the image sensor, so it has to be forced or squeezed into the frame.
  • Pincushion distortion - when straight lines in an image start to bend inward from the edges of the frame. This occurs in zoom or telephoto lenses and is the opposite of barrel distortion, in that the field of view is smaller than the image sensor and thus must be stretched to fit the frame.
  • Moustache distortion - this is a combination of pincushion and barrel distortion, where the straight lines of an image bend outward from the centre, and then inward from the corners of the frame. This creates a wave or moustache-like effect.
  • Lens type distortion - rectilinear lenses, such as wide-angle primes, tend to make lines in an image appear straight. On the other hand, curvilinear lenses, such as fisheye lenses, dramatically bend straight lines in an image. Both are deliberate forms of distortion.

    Perspective Distortion
This is where objects in an image can appear larger or smaller than they actually are, based on the distance between the object and the lens. This can be witness when people take images attempting to ‘hold’ the Leaning Tower of Pisa, where the subject stands closer to the lens while the object is further away, making the subject appear larger than they actually are.

Although many types of distortion add to the creative look of an image, most distortions are not particularly welcome by photographers. For this reason, many camera models have built in distortion correction, while post production software, such as Canon’s own Digital Photo professional, also have automated ways to reduce the effects of distortion.