As anyone who has looked through a window knows, glass both reflects light and lets light pass through. And reflections can easily happen at oblique angles as well as when you look at a lens straight-on. Camera lenses suffer from the same basic problem of reflections as windows. And excessive reflections within the lens can result in lens flare - either a generalized loss of contrast or bright reflected blobs appearing in the picture.

The invention of lens coatings by German lensmaker Carl Zeiss in the mid 1930s revolutionized lens design. Such coatings are fine layers of transparent material which are applied to the surface of lens glass and which minimize internal reflections within the lens. All modern camera lenses, including Canon EF lenses, are multicoated to reduce reflections. Canon refer to their technology as SSC, or Super Spectral Coating.

You can easily tell a coated glass surface from an uncoated one. An uncoated piece of glass reflects a lot of light, and reflected white light is also white. A coated piece of glass, however, reflects far less light and the light that is reflected is often greenish, purplish or reddish. These apparent colours, it should be noted, are artefacts of the way coatings absorb reflections, and do not mean that photos taken through decent quality coated glass are going to be tinted one colour or another.

Lens coatings have two drawbacks. First, they must be kept scrupulously clean at all times, as oils and dirt interfere with the way they work. Fingerprints are extremely obvious on coated glass. Second, many lens coatings are fairly fragile and are easily scratched, so care is required in handling and cleaning them. Some lens and filters include hardened surfaces over the top of the coatings to protect them, but this is not universal.