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Re: [OM] True, or just BS?

Subject: Re: [OM] True, or just BS?
From: Kennedy McEwen <rkm@xxxxxxxxxxxxxxxxxxxx>
Date: Mon, 7 Jul 2003 09:05:14 +0100
In article , Albert <olympus@xxxxxxxxxxx> writes
Moose,

I've read that coating off the front is not that big of a deal, but
coating off the rear element is.  I don't even claim to know the math
behind it or how to calculate it..

You probably don't need to understand the maths as much as the basic physics, but having understood the latter, the former is fairly obvious, if complex. The light going through a lens is either reflected off of one of the lens surfaces, absorbed or scattered within the volume of glass or transmitted. Ideally we would like all of the light to be transmitted as designed, because scattering and uncontrolled reflections reduce image contrast or cause spurious images - like the iris flare patterns you sometimes see in some contra jour lighting shots. Good glass minimises the scattering and absorption, leaving only reflectance from the surfaces to deal with, and that is where coatings come in. They are thin layers of material about 1/4 wavelength thick which creates two reflecting surfaces instead of one. You might think that would be worse, but the 1/4 wavelength thickness ensures that and reflection from coating surface is 180deg out of phase from the reflection from the glass, so the light waves interfere destructively and cancel each other out at particular wavelengths and angles of incidence. If the reflection intensity is the same from each surface then you get complete elimination of the reflection at that particular wavelength. Multicoating allows reflection at many wavelengths and incident angles to be eliminated. Thus coating reduces reflections to a minimum, which increases the light transmission and, more importantly, contrast dramatically.

From this it is fairly obvious that coating loss from the front element only affects two components in the light path - reflection of the light directly from the scene itself and secondary reflections from light reflected by internal surfaces in the lens. For the first component, the light is reflected back out to the scene, making no effect at all under most photographic conditions - although it can be significant in microscopy. The second component should already be low in any case due to the coatings on the internal lens elements, so loss of coating on the front surface has a small effect on an already low intensity. Thus, the effect of lost coating on the front element is much more cosmetic than optically significant.

On rear and internal elements this is not so, because the increased reflection from that surface can bounce off other surfaces in the lens - not just the coated optical surfaces - and find its way onto the film plane thus reducing the contrast of the image.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers

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