[rollei_list] Re: couple of ignorant lens design questions.
- From: Emmanuel.Bigler@xxxxxxxx
- To: rollei_list@xxxxxxxxxxxxx
- Date: Thu, 10 May 2007 19:30:23 +0200 (CEST)
> #1 Why is it not possible to add coatings to your lens by adding a
> coated filter? Or why can't Rollei come out with a plain glass with
> HFT coatings on it that you can put on your older lenses? I know
> that at least some filters are coated. Does it have the affect of
> adding a coating to an uncoated lens if you use one? My guess is
> that you can't put mulitple coatings on a single surface of glass so
> you have to coat multiple elements with different coatings to get
> all the HFT.
The answer is quite simple and is actually a non-obvious question so
it is good to ask it and tru to find some convincing answers in simple
words.
Usually everybody would say what "common photographic sense" suggests,
i.e. that any time you get an interface bewteen a piece of glass and
air, you add some parasitic reflections, you get more reflectiosn when
teh refractive index of glass is high. And modenr lens design loves to
use high refractive indices, combined to a variety of glasses of
course.
For example some modern ophtalmic eyeglasses have high refractve index
lenses, for people being, like me, severely short-sighted, the higer
index brings less distorsion and thinner, more aesthetic glasses. But
you have to add an anti-reflection coating on those glasses to avoir
parésitic reflections, hence a premium on top of already expensive
glass materials.
So if you take an old un-coated photographic lens with several lens
elements inside and add any kind of piece of glass, be it multi-coated
or not, you add more glass-air surfaces in the whole system hence you
add more parasitic reflections.
Well, one could argue that an anti-reflection coating, in a sense, is
a thin layer or combination of layers of transparent materials added
just close to an air-glass surface.
The question is in fact : how close it is. In order to cancel
reflections by a so-called interference phenomenon, the additional
layers should be very thin, a fraction of the wavelength of light,
i.e. a fraction of a micron, and deposited directely on glass. Adding
another separare piece of coated glass even one tenth of a millimetre
away from any lens element does not cancel anything but simply adds
more flare as any glass-air surface.
So all lens elements have to be coated.
--
Emmanuel BIGLER
<bigler@xxxxxxxx>
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