The difference between portrait lens and ordinary lens

The key to taking portraits is to have a small deformation. A lens with a focal length of 50mm is similar to the angle seen by the human eye, so it is called a "standard lens". When the focal length is a little longer (within 100mm), the deformation is small, but after 100mm, although the deformation is small in the plane direction (up and down, left and right), it is deformed (compressed) in the depth direction (front and back). Among fixed-focus and zoom lenses, the imaging quality of fixed-focus lens is much better than that of zoom lens. To sum up, it is best to use a fixed-focus lens with a focal length of 80 or 85mm for portrait shooting. The horizontal deformation of wide-angle lens is the largest (mainly edge deformation), and the smaller the focal length, the greater the deformation. This is a side effect of people's requirements for lenses. The test lens adopts equidistant plane grid diagram, and the photos taken are required to be consistent with the original without deformation. However, the wide-angle lens differs greatly from the center and edge of a square, which obviously violates the principle that the human eye observes that the near is big and the far is small, so that the deformation of the lens adapts to the unreasonable requirements of people. The photos taken must be the same person, the middle person is thin, the edge person is fat (horizontal photos), or the upper and lower faces are long, and the middle face is round (vertical photos) (in fact, the deformation of the center is the smallest, and the illusion caused by contrast with the surrounding is round).