Flare in an optical system results when stray light (non-image forming) comingles with the image-forming light rays. A lens hood diminishes the ill effects of flare by shielding the lens from unwanted peripheral light rays. Every picture will benefit if a lens hood is mounted. This is because stray light enters the lens barrel, and is reflected off the polished surfaces of each individual lens within the optical system. This stray light bathes the image sensor (or film), and this reduces the contrast of the visual image. Flare light has many sources -- thus mounting a lens hood won’t eliminate flare.

The chief countermeasures are flat-black paint applied to the inter surfaces of the camera and the lens barrel together with a coating on each lens. A thin mineral coat is applied to all polished optical surfaces. It is the thickness of the coat that does the trick when it comes to mitigating internal reflections within the optical system. A coat thickness of ¼ wave length of light will dampen reflections. Often each lens will have multiple coats, one for each color (frequency) of light.

In summation, flare is devastating; it robs our images of contrast. The lens hood is just one additional measure we can bring to bear.