Daniel:
Coatins on lenses have nothing to do with "filtering" the light. The
coatings are actually interference coatings (the thickness of the
coating is some fraction of a wavelength of a given color of light,
typically 1/4 wave of 5550 angstroms for SC coatings). These coatings
act to increase the transmittance of the lens. Uncoated glass reflects
about 40f the light hitting it due to the difference in the indicies of
refraction of air and the glass (1.0 for air, 1.5 for normal glass).
Put very simply, the interference coating reflects the light that reflects
off the glass back into the glass, thus increasing transmission.
(In actuality it is more complicated than that.... see the book
"Thin Film Optical Filters" by Angus MacLeod if you want to know the
physics behind it all).
The problem is that an SC can only correct well for one wavelength of
light, since a single coating can only be the proper thickness for one
wavelength (let's say 5550 Angstroms). Thus, the average transmittance
for a single coating is about 990er surface.
If you were paying attention, that last sentence would have made you sit
up and think... hmmm... a lens I use a lot, the 35-105, has 16 elements
in 12 groups. This means there are 12 x (2 air-glass surfaces/group) for
a total of *24* air-glass surfaces in this lens (you can ignore cemented
glass to glass surfaces, since the cement matches the indicies of
refraction between the elements).
Light loss by reflection is multiplicative. This means that if the
first surface transmits 0.99 (or 99%) of the light, the second surface
will transmit 0.99 * 0.99, or 0.98 (98%) of the light. For a single
coated lens, the amount of light transmitted by the last surface to the
film, assuming 24 air-glass surfaces, is 0.99^24 (0.99 raised to the
24th power), or about 79 percent. This is much better than an uncoated
lens, which in this case would only transmit 0.96^24, or about 38
percent of the light to the film.
The even bigger problem is loss of contrast from stray light. That
reflected light can get reflected back to the film eventually, as an out
of focus image (if you were taking a picture of a point source like a
star, you would get a lot of out of focus diffuse circles on the film
which would lower the contrast of the final image).
While SC is obviously better than no coatings at all, let's look at MC:
A good broadband (meaning from the blue to the red in the optical
spectrum) consists of several coatings to maximize transmission in
several wavelengths. This results in a transmission of something like
0.999 (or 99.9%) per surface. For a 24 air-glass surface lens, this
means 0.999^24, or about 980f the light hitting the lens is
transmitted to the film. This is a huge gain over SC in throughput, but
more importantly, there is a lot less reflected light bouncing around
lowering the contrast of your images.
That's why you want MC in a (very big) nutshell.
Cheers,
--Lee
________________________________________________________________________
R. Lee Hawkins lhawkins@xxxxxxxxxxxxxxxxxxxxxxxxxx
Observatory Engineer hawkinsrl@xxxxxxxxxxxx
Dept. of Physics & Astronomy http://brashear.phys.appstate.edu/lhawkins
Appalachian State University Ph. 828-262-7331
Boone, North Carolina 28608 FAX: 828-262-2049
________________________________________________________________________
< 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 >
|