Introduction to dark corner

The word dark corner belongs to photographic terminology. The phenomenon that the four corners of a picture darken against a scene with uniform brightness is called "loss of light", commonly known as "dark corner". Dark corners are inevitable for any lens. The main reason for the dark corner is: 1. There is a large angle between the imaging light at the corner and the optical axis of the lens, which is the main reason for the loss of light at the corner. When you look at the diaphragm at the edge of the field of view along the direction of light, because the light has an included angle with the plane where the diaphragm is located, the diaphragm you see is oval, so the light passing area is reduced. The distance from the optical center of the lens to the edge of the film is large, and the angle between the light rays and the negative film with the same aperture diameter is small, so the brightness will inevitably decrease. Similarly, the same deflection angle of light, for corner light, the displacement is large, which is equivalent to a large irradiation area. The area is proportional to the square of the displacement, so the edge brightness is proportional to the fourth power of the COS value of the angle between the light and the optical axis. In other words, the edge brightness of wide-angle lens drops sharply with the increase of viewing angle. 2. telephoto lenses, especially zoom telephoto lenses, have many lenses. In order to let the corner light pass through, these lenses must be very large. In order to reduce the cost, the diameter of these lenses is reduced, which leads to the incomplete passage of corner imaging light and reduces the brightness of corners. There is a big deviation in the corner. In order to improve the imaging quality, the edges of some lenses or specially set apertures deliberately block some edge rays that affect the imaging quality, resulting in light loss at the corners. Although the fisheye lens has a large viewing angle, there is almost no corner loss because of the small edge magnification. High-end zoom lenses cost a lot of money, which can enlarge some lenses and correct aberrations perfectly. High-end telephoto lenses, including zoom lenses, have little edge loss. Also: If a wide-angle lens uses too many color filters, it is equivalent to adding a lens barrel, which may cause dark corners or even black corners!