From the common sense of optics, when light passes through a lens, a part of the light is reflected and absorbed, accounting for 5% and 2% respectively.
The lens is composed of lens groups. If the light is reflected and absorbed so much on each lens, it is very few that the light passes through the lens group and reaches the focal plane, that is to say, the light is seriously lost when penetrating the lens.
Moreover, the reflection between the lenses will seriously affect the final image quality. When the light penetrates each lens, it is also reflected continuously by the surface of each lens. Some of the reflected light shoots out of the lens, and some of the reflected light is reflected by the surface of other lenses in the lens, and finally falls on the focal plane, which produces glare in the image. As a result, the contrast of the image, especially the shadow part, decreases, the haze increases, and the sharpness of the image is greatly reduced.
In order to improve the transmittance and image quality of the lens, the lens must be coated in the modern lens manufacturing process.
According to the principle of optical interference, the lens is coated with a quarter wavelength material (usually fluoride) on the lens surface to minimize the reflection of color light at this wavelength. Obviously, one layer of film only works on one color light, while multi-layer coating can work on multiple colors. Multilayer coating usually uses different materials to coat the lens surface with different thickness. Multilayer coating can greatly improve the transmittance of lens. For example, the reflectivity of each surface of uncoated lens is 5%, which is reduced to 2% after single-layer coating, and can be reduced to 0.2% by multi-layer coating. In this way, the diffuse reflection between lenses of lens can be greatly reduced, thus improving the contrast and sharpness of images.
But the coating is easy to scratch.