What is lens distortion?

Lens distortion is actually the general term for the inherent perspective distortion of optical lenses, that is, the distortion caused by perspective, which is very unfavorable to the image quality of photos. After all, the purpose of photography is reproduction, not exaggeration, but because this is the inherent characteristic of the lens (convex lens condensing, concave lens diverging), it can not be eliminated and can only be improved. The optical design and materials of high-grade lenses are exquisite. Using the optimized design of lens group and selecting high-quality optical glass (such as fluorite glass) to make lenses can reduce the perspective deformation to a very low level. However, it is impossible to completely eliminate the distortion. At present, the best quality lenses are tested under extremely strict conditions, and the edges of the lenses will be deformed and distorted to varying degrees.

Close-up photos taken with a wide-angle lens, in which the subject's nose will appear surprisingly large compared with other facial organs. This is a feature of perspective distortion in many photos taken with wide-angle lenses.

The reason for this distortion is actually perspective, normal perspective. As we all know, one way for eyes to feel distance is to use the relative size of objects, that is, "the near is big and the far is small". In photography, the perspective relationship is also expressed in the same way: parallel rails will get closer and closer as we look far away until they converge. The essence of this phenomenon is that the distance between rails becomes smaller on the surface.

Another manifestation of perspective is that the closer the object is, the stronger the perspective effect is. For example, 200 soldiers marched in a row. If you watch or photograph the parade at a distance of 10 feet from the soldier in front, the soldier in front will appear much taller than the last soldier. However, if you watch or photograph the same team at a distance of 100 meters from the soldier in front, the size difference between the first soldier and the last soldier will not be so big.

These two features of perspective also apply to all shots, namely:

1. The farther the subject is, the smaller it appears;

2. The farther the lens is from the subject, the smaller the appearance change of the subject.

2 Distortion of different purposes

Perspective distortion of wide-angle lens

So why do wide-angle lenses often produce distorted perspective relationships, such as the root of the weird nose in the example? Because wide-angle lenses are often shot very close to the subject, the closer the shooting distance, the stronger the perspective effect.

If all lenses are used at the same distance, the wide-angle lens will not distort the perspective effect more than any other lens. In fact, it is not difficult to prove this through experiments. Shoot a row of columns or a row of trees or any row of objects with lenses with different focal lengths. All the photos are taken at the same position, and then enlarge the same part of each image in order to get the same size image in the photos. Finally, no matter what the focal length of the lens is, the perspective effect of any photo will not be different. The reason is that the shooting distance of all photos is the same, that is, the distance from the subject to the lens is the same.

Now, let's go back to the problem of big nose in close-up photos. The tip of a person's nose is about 1 inch closer to the lens than other parts of the face. Because the closer the subject is, the bigger it will appear, so the nose will appear disproportionately larger than other parts of the face when it is close to shooting. So, why does a wide-angle lens make this distortion more obvious? Because in order to make people feel full of pictures, the wide-angle lens must be very close to the subject. For any kind of lens, this distortion will occur when it is very close to the subject to a certain extent. The closer to the main body, the more serious the distortion. It is precisely because I want the subject to fill the picture that I have just entered the distortion range of the wide-angle lens.

Perspective distortion of telephoto lens

In fact, as the subject gets farther and farther away, the perspective distortion will become smaller and smaller, but it will begin to flatten, that is, it will lose its level and details. Two objects far apart look as if one is on top of the other. This is a kind of reverse distortion, which often appears when shooting with a telephoto lens. Because the subject is very far away from the camera, it produces a plane perspective effect.

Why do photos taken with telephoto lenses often appear this way? This is because when using a telephoto lens, the shooting distance is often longer. In fact, using any lens at the same distance will produce this distortion.