How are dark corners formed?

The dark angle of a wide-angle lens is determined by the principle of geometric optics.

Assuming that the half-field angle is ω, the corner illumination is proportional to the fourth power of cos(ω). The reason is:

1。 The entrance pupil of the lens is round, but the entrance pupil seen by oblique light is oval (first order);

2。 The light passing through the lens shines obliquely on the negative or CCD (main surface);

3。 The distance from the lens center to the bottom film or CCD edge is longer than the center, and the illuminance is inversely proportional to the square of the distance (quadratic).

So the dark angle of wide-angle lens is inevitable! BS itself, Baidu players. The following is my answer. Photography is imaged by the reflection of a photosensitive element or film through a lens. It can be understood as an X-shape, in which the center point of X is the lens, the photosensitive element on the left and the object on the right. When the right viewing angle is wide, the lens hood or lens edge blocks light.//\-Similar to the effect in the above picture, the horizontal line is the hood or lens edge. Then the data obtained by the photosensitive element will lack edges and produce dark corners, which can be achieved by increasing the focal length, removing UV and removing the hood.