TECH­NI­CALLY SPEAK­ING

THESE LOVELY LAY­ERS ARE MORE THAN JUST A PRETTY FACE

Shutterbug - - Contents - By Seth Shostak

The Real Rea­son for Lens Coat­ings

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 ref lects some light, for ex­am­ple the flu­o­res­cents on the ceil­ing.

Clean wa­ter’s pretty darn trans­par­ent, but it still re­flects some light. The glass in your cam­era op­tic is also trans­par­ent, but it typ­i­cally re­flects 3 or 4% of any in­ci­dent light, no mat­ter how much you clean it. When a light ray passes out the op­tic’s back end, the same physics ap­plies, and you lose yet more light to re­flec­tion.

For a sim­ple lens, the to­tal light loss from re­flec­tion is maybe 7%, which is to say that about 93% of the light ac­tu­ally makes it through the op­tic. No big­gie. But roughly 180 years ago, Joseph Pet­z­val made a very suc­cess­ful cam­era lens hav­ing mul­ti­ple el­e­ments. That started a trend, be­cause more el­e­ments gen­er­ally pro­duce faster, sharper lenses.

Con­sider what re­flec­tive light loss would mean for a four-ele­ment op­tic, just as il­lus­tra­tion. The frac­tion of trans­mit­ted light would be 0.93 x 0.93 x 0.93 x 0.93 = 75%. To­day’s zoom lenses might have 10 to 20 glass el­e­ments, and, lack­ing coat­ings, only 48% of the light would make it through to your sen­sor—a loss of more than one f/stop. More el­e­ments, more loss.

There’s an­other prob­lem. Much of the 7% re­flected light bounc­ing off each ele­ment ul­ti­mately ex­its the rear of the lens, and fogs your sen­sor. Blacks are no longer black, and con­trast and dy­namic range won’t rise to the level of medi­ocre.

The punch line? With­out coat­ings com­ing to the res­cue to mit­i­gate re­flec­tions at glass-air sur­faces, mod­ern lenses wouldn’t be prac­ti­cal. Highres­o­lu­tion, mul­ti­ele­ment op­tics would be a pipe dream for an­other uni­verse.

But how do the coat­ings work? De­spite the ap­pear­ance of your lenses, these lay­ers are ac­tu­ally col­or­less, and usu­ally made of mag­ne­sium flu­o­ride. But they’re very thin. In­deed, they are 1/4 the wave­length of the light whose re­flec­tion is to be mit­i­gated. They also have to be in­ter­me­di­ate in re­frac­tive power be­tween glass and air.

So imag­ine a ray of light im­ping­ing on a coated lens. Some of that light is re­flected from the coat­ing, and ad­di­tional light is re­flected from the glass be­neath the coat­ing (be­cause the coat­ing and the glass have dif­fer­ent re­frac­tive power). But the ray sub­ject to the sec­ond re­flec­tion trav­els far­ther: an ex­tra 1/4 wave­length com­ing in, and an ad­di­tional 1/4 wave­length go­ing back out. The to­tal ex­tra path length from the sec­ond re­flec­tion rel­a­tive to the first is 1/2 wave­length. That’s 180 de­grees of dif­fer­ence, and the two rays can­cel out! Like magic, all the light of that wave­length passes through the lens.

While this clearly elim­i­nates the re­flec­tion loss, it does so only for one wave­length.

But ad­di­tional coat­ings can ad­dress the prob­lem for other col­ors, and the best mul­ti­coated lenses will have re­flec­tions that have been cut to un­der 1% from deep red to deep blue. That’s greater trans­parency than the Nor­we­gian gov­ern­ment.

Coat­ings are more than cos­metic. They’ve made mod­ern lenses pos­si­ble.

But in a fu­ture col­umn, we’ll dis­cuss some new tech­nol­ogy that may make tra­di­tional coat­ings as ob­so­lete as flash pow­der.

Seth Shostak is an as­tronomer at the SETI In­sti­tute who thinks pho­tog­ra­phy is one of hu­man­ity’s great­est in­ven­tions. His pho­tos have been used in count­less mag­a­zines and news­pa­pers, and he oc­ca­sion­ally tries to im­press folks by not­ing that he built his first dark­room at age 11. You can find him on both Face­book and Twit­ter.

Some of the au­thor’s abused lenses. Note that all show the pas­tel col­ors caused by anti-re­flec­tive coat­ings.

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