What is distortion?

When an image does not match reality, it’s distorted. There are several ways a lens can distort your image ans some are associated with the design of the lens, while other types of distortion are caused by the photographer.
Distortion of an image doesn’t necessarily have to be visible to the naked eye. Usually, you’ll encounter horizontal and vertical geometrical distortion. These two types of distortion are no lens errors, but have to do with the perspective from which the photo is taken. Vertical and horizontal distortion are caused by the orientation of the camera relative to the subject.

Barrel or pincushion distortion is caused by lens errors. Less than 1 percent of barrel or pincushion distortion is no longer visible in most photos, except for architecture images. With a fisheye lens, this distortion is very large, but deliberately present.

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Distortion caused by lenses
Pincushion distortion When the points of an image is stretched and the middle is pressed together, we call it pincushion distortion. If you see a positive number for distortion in the graphs of our lens test based on Imatest, then there is pincushion distortion. This type of distortion is common with telephoto lenses.
Barrel distortion When a picture appears to be more rounded than the original, it is because of barrel distortion. If there is a negative number for distortion in a graph of our lens test based on Imatest, we are dealing with barrel distortion. Wide angle lenses often exhibit barrel distortion.
Barrel or pincushion distortion can be corrected using software, but not simultaneously. And it happens that a lens has both barrel and pincushion distortion. A straight line at the edge of the image will appear slightly wavy: lightly barrel in the middle barrel and lightly pincushion in the corners. This combination of distortion cannot be corrected with software. With zoom lenses, distortion depends on the focal length and often goes from barrel to pincushion. There are specific lens correction profiles available for Panorama Tools, Photoshop, and Adobe RAW converter, which allows you to correct all shots batch-wise for these lens errors. Although it is technically possible, it remains annoying to correct the distortion resulting from lens errors with software. Therefore camera manufacturers start to offer the opportunity to already correct barrel and pincushion distortion in the camera.
Geometrical distortion
Vertical geometrical distortion If you are in front of a tall building and you hold your camera up slantwise, you’ll get an image where the vertical lines convert. This is called vertical geometrical distortion. This is not caused by a lens error, but oroginates from a perspective distortion caused by the photographer.
Vertical distortion is probably the most common type of distortion in photography. You can use software to correct it, but you will have to do so manually for each separate image. The software cannot guess how much you have tilted the camera when you made the image and whether that was intentionally or not. For these situations, special tilt-shift lenses are for available, with which you can take pictures without vertical distortion.
Horizontal geometrical distortion If you don’t hold your camera perfectly parallel to the subject that you’re photographing, there will be a horizontal geometrical distortion. This wil occur when you take a picture of a building, while you do not stand exactly in the middle in front of that building.
You can recognize this type of distortion, when you are unable to straighten out all the horizontal lines in Photoshop. In the picture above, the roof of the building is set horizontally in Photoshop, but the curb is very skewed, while this is not the case in reality. As with vertical distortion, this can be corrected with software, but you will have to correct each image separately.