face WitchDoc access_time Wed Jun 12, 2019

The magic of lens coating

What is that weird color on the front of your camera lens? If you look carefully you would find different colors on the surface of the camera lens. But if you look through the lens you will not find any color! Certainly not in a good lens. It is all clear and colorless. How come a clear lens has color on the surface?

That colored thing is the anti reflection coating on the lens. You can find that coating on good quality binoculars too.

In every day life, in monsoon, you would find that phenomenon on the oil slicks on the puddles in the road. A thin layer of oil produces such a color on the water surface. Those changing colors are something beautiful to watch.

You can also see that on some spectacle glasses.

What does that anti reflection thing do?

The name gives away the secret.

It reduces the reflection or what photographers call flare.

When light enters the lens, a little bit of it is reflected back. But most of the light goes through the glass. The next surface that comes in the path of the light is the back of the glass element.

Since there there is a density difference between the glass and what ever is behind it, a little bit of the light reflects backwards, that is to the front of camera. But this reflected light hits the back of the front surface of the lens. There again it gets reflected back into the camera. This game of reflection goes on till the light is completely killed.

This reflection back and forth, produces degradation of the image in the camera. The reflected wave of light interferes with the incoming light, because they are of the same wave length.

During the reflection the peak of one wave gets slightly delayed and where there should have been a peak there would be a trough.

This trough cancels out the peak of the wave that comes in its way.

That is, that wave of light kills its comrades. Moreover, the reflected light takes a different path and forms a blurred image on the sensor or film in the camera.

What ever happens, it is not for our good.

The solution the scientists and engineers found out was to use the same reflection to kill that reflection.

So they gave a transparent, but reflective coating of very specific metallic compounds on all the glass surfaces.

The depth of this coating is very critical. The depth is exactly half the wave length of the light we want to kill. Since it is half the wave length, the troughs get shifted to peaks and the reflections are canceled by the interaction of troughs and peaks.

But the catch is that, every shade of light has its own wave length. That means we need to have more than one coating to neutralize all the reflections. Some lenses mention that they are multi coated. What they mean is they have different coating for different wave lengths.

This coating actually increases the contrast, saturation and sharpness of the image by reducing the ghost images.

It is a good thing generally.

But there are photographers who go hunting for lenses that are not coated.

The really old lenses!

But who wants a lens that has low contrast, low saturation and low sharpness?

Strangely, there are people who look for that. Those "poor quality" lenses are ideal for some portraits. It gives the portrait an other worldly look! There would be a halo around the person. It is great especially for portraits of ladies.

But if you want to get that effect, you do not have to peel off the coating on your expensive lens. All that you have to do is to get a cheap, clear ultra violet filter, smear a thin layer of oil on the front of the filter and use that filter in front of your camera lens. For heaven's sake do not smear that oil on the lens.

What can kill that anti reflection coating?

There are very many devils trying to kill that coating.

A thin layer of any fluid can alter the thickness of the coating and can render it ineffective. For example thin layer of grease or oil. You may not even realize that there is oil on the lens. That will kill the lens coating. That will change the depth of the coating.

That coat of oil comes not from an oil bath! Or did you give your lens an oil bath?

It comes from your fingers! Yes your fingers. There is oil on your fingers all the time. Never ever touch the lens.

Scratches, really fine ones, can spoil the coating.

Moisture can do the same harm. Sea spray, salt and lens coating are not exactly friends. Keep them apart.

The easiest way to keep the lens surface clean and safe is to use a good quality UV filter in front of the lens. But some photographers shudder at the thought of of a UV filter.

A filter, any filter, in front of the lens can produce a whee bit of loss of image quality. But you decide if you want to buy a new lens every so often for the sake of an almost imperceptible quality of the image.

There is practically no way to re coat that precision lens. That is too high a technical job to be done any where out side the haloed precincts of the lens factory. I do not think the lens maker would oblige you for love or money.

Have you any idea what are the materials that are coated on the lens? I have no idea.

So take good care of your lens.

It is precious.

Long back, in the era of color photographic printing, the filters in the color enlargers were made with this type of coating. Those filters were required to tolerate high temperatures and yet maintain the color accuracy for long time.

No color dye could do that. They have the technical name dichroic filters. Needless to say a small scratch can kill that filter just like a camera lens. Fortunately very few users had access to that filter hidden deep inside the enlarger.

That color on the lens is our friend. Take good care of our friend, keep that friendship for ever.

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