What does the coating on the photographic lens do?

The coating of lens is called antireflection film.

The main function is to reduce the reflection on the glass surface and reduce the attenuation of light.

This is very important in modern lenses, the reflection on the glass surface is reduced, so that the reflection on the glass surface on the zoom lens of more than a dozen lenses will be much less, so that the light passing through the lens will not be significantly reduced. Another advantage of antireflection after coating is that it is not easy to produce backlight fog. The performance of the lens under backlight condition is obviously improved.

Early lenses were coated and uncoated, and some simple lenses were also coated with a single layer. Modern lenses are all multilayer coated. However, if the coating fails to reach the pass rate of 100%, it will still reflect some light, so the lens will have various colors. The lens on the lens will not totally reflect one color of light, so the lens you see will often reflect various colors.

A physics teacher in high school once asked me what color the lens reflected, and I replied that there were all colors. She shouted impossible (staring at her eyes and telling lies); I think what your teacher said is also reasonable, because the previous lens was not divided into several groups like the current lens, such as focus adjustment group, zoom compensation group and so on, and some of them were moved to the direction group. . . . . . My high school physics teacher also asked me what color the lens of the camera lens was, and I replied that it was purple. The reason should be known to all, that is, because of the antireflection film, the transmittance of visible light is enhanced, and the purple with higher frequency is reflected back, so the lens will appear purple or deep purple. . . . . .