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
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|