Common Lens Distortions
Optical distortions are very common to all camera lenses due to the shape and give lenses their own characteristics. Mostly, they are rather lens characteristic than fault. Barrel and pincushion distortions are the most frequently used words in Lens distortions. Third one, the worst kind, is the mustache distortion. As long as they are in acceptable level, you don’t need to worry about it. Some people actually like them and use them creatively.
No Distortion
Barrel distortion
Here the straight lines curve inwards as it moves away from the optical axis and shape of the curves which looks like a barrel. Hence called barrel distortion. This happens when the image magnification decreases as the distance from the optical axis increases. barrel distortion happens because the field of view of the lens is much wider than the size of the image sensor and hence it needs to be “squeezed” to fit. Commonly seen on wide angle lenses. Fisheye lens is a perfect example of barrel distortion, which makes a photo look bulged outwards at the centre of the image.
Barrel distortion
Pincushion distortion
In barrel distortion an image has a shape of a barrel; in case of a pincushion distortion an image takes the shape of a cushion, i.e. as the straight lines move away from the optical axis they curve outwards. This phenomenon is seen in telephoto lenses.
Pincushion distortion
Mustache Distortion
Mustache Distortion contains both barrel and pincushion. At the centre of an image a barrel distortion appears and gradually turns in a pincushion shape at the edge of the same image. Probably the worst kind of lens distortion.
Mustache distortion
Both the barrel and pincushion distortions are quadratic. This means that their distortions are proportional to the square of their distance from the centre. But in mustache distortion in the centre it is quadratic due to the barrel being the dominant distortion, but at the edges it becomes quartic (to the power of 4) as both barrel and pincushion distortions are visible.