Most of the light entering a lens passes through the lens elements to the film or sensor, but a small percentage is reflected at each air-to-glass and glass-to-air surface. This occurs in all lenses but is more likely to cause noticeable problems when the front and back elements are soiled, or when lenses with wide apertures are directed too close to the sun. The internally reflected light reduces colour saturation and contrast, and may produce irregular smudges of colour across an image. The effects generated by non-image-forming light are collectively known as flare.
Various materials such as silicon dioxide are used to coat lens surfaces and so reduce the amount of light reflected. Modern multi-coating processes are particularly effective but none can completely eliminate the problem. Using a lens hood, or shading the lens with your hand, reduces flare by shielding the lens from light originating outside the picture area.
Bright highlights or points of light in the frame are particularly likely to cause flare. Light reflected back through the diaphragm, and then back again to the film or sensor, can produce numerous out-of-focus images of the diaphragm aperture in an image. The acceptability of the phenomenon is dependent upon the number, size and colour of the spots and their placement in the picture. In some circumstances the effect can be tolerated, but it is generally better avoided.