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Re: [OM] The SC MC Debate again

Subject: Re: [OM] The SC MC Debate again
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Tue, 04 Dec 2001 23:27:02 +0000
At 03:26 12/5/01, Damon Wood wondered:
What sort of situations (due to light of course) should I use it, in preference of my SC 50mm 1.8. I am thinking along the same conditions as the ones you presented with the wide angle lenses.

To my knowledge of physics, would I be correct saying (in some conditions) that the SC has advantages with skin tones rather than MC lenses. Also, when applying the MC lenses to night photography, how should they be used, if at all.

Cheers again guys,

Good material on the OM listing at the moment. Keep it up guys.

Damo

Damon,
Don't think of AR optical coatings as "filters." They're not. They work by creating an interference with the light waves passing through them. The single-coating model on the front objective is probably the easiest to explain. The single coating has an index of refraction between the glass it coats and air. As light strikes the coating, some of the light is reflected from the coating. The remainder passes through to the next interface of coating and glass. Some of the light reflects from this interface also and passes back out past the first air-coating interface. The coating index of refraction and its thickness is selected so that the light waves from the two reflections are now 180 degrees out of phase and "cancel" each other. "Conservation of energy" states that it cannot simply disappear! Nor is it absorbed. Where does it go? Past the second coating-glass interface and into the glass. This is how an AR coating improves light transmission through the optics it coats. Because various ray paths are at different angles, and a coating is only good over a portion of the visible spectrum, it's not 100 0.000000e+00ffective. Thus, light transmission into the glass is not 100%, but for a single coating it's improved quite significantly compared to no AR coating.

For a single coated lens, the coating works optimally in the yellow-green center portion of the spectrum. Multi-coatings have the centers of each coating spread across the spectrum (e.g., for three coatings, one would be in the upper red, another in the green and the third in the lower blue). This improves transmission better across the entire spectrum, and this is why a multi-coating makes a slight improvement over a single coating, but not nearly as much as a single coating does compared to no coating. It also improves contrast slightly.

I suppose you might be able to measure the slight difference in spectral transmission between single and multi-coated lenses of the same formulation using sophisticated optical lab equipment. However, in practical application I have yet to notice the difference except in very slightly better flare control and then it's only under very difficult lighting conditions prone to extreme flare. The one SC lens I have the most experience with under high risk of flare is the 35/2.8 Zuiko Shift which I've used for night photography in urban areas with many, very bright pinpoint sources of light. No objectionable flare has ever been observed. Refraction around the corners where the aperture blades meet causes a much more noticeable star effect, the points of which are still very, very small compared to the star filter I have.

IMHO, much more ado about SC versus MC is made than the benefit provided by an MC lens. There are aspects of the lens design that have a much greater effect on contrast and flare resistance than going from SC to MC.

-- John


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