The Real Reason For Lens Coatings

These Lovely Layers Are More Than Just a Pretty Face 

- Seth Shostak

AMONG THE MOST AESTHETICALLY satisfying features of a good lens is the lovely color of the front glass. Check out the ads trying to lure you to new optics, and you’ll see come-hither hues of blue, purple, or yellow—as tempting as a new car’s snazzy finish.

These very thin (five millionths of an inch) coatings are certainly attractive, but are they utilitarian? After all, few lenses made before 1941 had ’em, and those optics worked just fine.

Well, they didn’t work all that “fine.” Don’t sell coatings short. Without them, your lens collection would be small.

Here’s the problem: when light passing through air encounters some other transparent medium, it both bends (refracts) and bounces (reflects). The bending is what a camera lens is all about—it’s how the lens forms an image. Fill a glass with water and stick a spoon in it. Then look at the glass from the side and check out where the spoon enters the water. Refraction! You’ll suspect that Uri Geller has been messing with your flatware.

But look at the glass from above, and you’ll notice that the water also reflects some light, for example the fluorescents on the ceiling.