What is Distortion?

Image distortion comes in two different forms. Sometimes it’s easy to spot distortion and other times it can be much more difficult to see. Optical distortion is caused by the lens and is sometimes referred to as lens distortion. The second type of distortion, perspective distortion, is caused by position of the camera in relation to the subject.

It’s important to be able to identify and differentiate between both types of distortion in your images. Even the best lenses have distortion. As much as manufacturers try to create lenses that are perfect, it’s impossible to make a lens that has absolutely no distortion. In this article, we will be covering both types of distortion in detail.

Optical Distortion


Optical distortion is a result of lens design. It is most easily seen in images that contain straight lines. The optical distortion causes lines to deform and makes them appear to be curved or wavy. This problem is usually more pronounced in wide angle lenses and some zoom lenses.

Optical distortion can manifest in three different ways: barrel distortion, pincushion distortion, and wave distortion (or “moustache” distortion).

The grid below is an example of an image that has no distortion. Notice how the lines all appear straight and uniform.

no distortion

Barrel distortion makes the image appear to bow outward. The image magnification is greater in the center and decreases toward the outer parts of the image. This causes the lines to bow slightly inward toward the center. If you look at the example below, you can see how the grid appears like it could be wrapped around the outside of a barrel.

barrel distortion

Pincushion distortion makes the image bow inward, making the image appear “pinched”. The image magnification is greater toward the corners of the image making the center of the image appear to bow in. The lines in the image appear to bow out toward the corners of the frame.

pincushion distortion

Wave (Moustache) distortion is the worst kind of distortion and is much harder to fix than the other types. It is a combination of barrel and pincushion distortion. The center of the image is magnified like barrel distortion creating lines that are curved inward. Towards the corners of the image pincushion distortion occurs and causes the lines to curve outwards, thus creating wavy or “complex” lines as you can see below.

wavy distortion

Perspective Distortion


Perspective distortion is all about placement and distance. It has nothing to do with the design of a lens. Lenses work the same way as your vision does, objects that are closer appear larger than a far away object. Images take three dimensional objects and flatten them onto a two dimensional space. If the object is too close or too far from the camera, it can appear disproportionate to the rest of the objects in the image.

The tracks are largest when the distance is closer to the camera.  As they move into the distance, they get smaller.

The tracks are largest when the distance is closest to the camera. As they move into the distance, they get smaller.

Perspective distortion has two different forms: extension and compression distortion. Extension distortion happens when an image is taken with a wide-angle lens from close distances. Extension distortion renders close objects disproportionately large and and distant objects disproportionately small.

The puppy's nose is disproportionately larger than the rest of his face due to distortion

The puppy’s nose is disproportionately larger than the rest of his face due to extension distortion

Images of people can look particularly distorted when taken from close distances with a wide-angle lens. Just like the image of the puppy above, their face can become distorted. A person’s nose will look disproportionately large compared to the rest of their face, while their ears may look too small or disappear all together.

Compression distortion (also called telephoto distortion) occurs when shooting subjects from a distance with a telephoto lens. Objects that are close appear disproportionately smaller and objects much further away appear disproportionately large.

A common misconception is that the focal length of a lens causes perspective distortion. Changes in perspective are caused by the physical distance and not the lens. If we were to take three lenses (wide, normal, and telephoto) and shoot an object from the same distance, all three images would have the same perspective. The image shot with the telephoto lens would make the object look larger in the frame and the wide-angle lens would make the object appear smaller, but if you take all three images and zoom in to the same spot in each, the perspective would be identical.

The reason people tend to get confused is because wide-angle lenses are typically used from close distances and telephoto lenses are used from further away. Changing the distance is what changes the perspective.

Converging Lines


Images can also display what’s called converting lines. If the camera’s sensor (or focal plane) is not lined up completely parallel to the subject, the subject will have a “leaning” effect.

The image below taken of two buildings has converging lines. When I took this image, the camera was angled up to fit the top of the buildings into the frame. Notice how the top of the buildings appear much smaller than the base. They also appear to be leaning away from the camera.

perspective

If I had pointed the camera directly at the buildings, the distortion would not have occurred. Architectural photographers use special PC (perspective control) lenses that can shift the focus plane and reduce the leaning effect.

perspective-distortion-diagram

As you can see in the diagram above, the image to the left shows the camera pointed directly at the building. The focal plane is lined up parallel with the building’s surface, so the image will not appear distorted. The scene on the right is similar to the image of two buildings from above. The focal plane is not lined up parallel to the building and there are converging lines.

The red lines show why this occurs. Note that, on the left, the lines are both the same distance. On the right, the line from the camera’s sensor to the top of the building is much greater than the distance to the base of the building. This causes the top of the building to appear much smaller and makes the building look like it’s leaning away.

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