[pure-silver] Re: Optical question
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 29 Feb 2008 22:22:33 -0800
----- Original Message -----
From: "Peter De Smidt" <pdesmidt@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, February 29, 2008 5:00 PM
Subject: [pure-silver] Optical question
Suppose one were to put a piece of 5mm thick acrylic
between a negative and an enlarging lens. Would the
piece of acrylic cause the distance needed to focus the
negative at a given size to change? If so, would there
need to be more or less extension between the negative and
the lens?
It would cause a small difference. The distance would
depend on the ratio of the index of refraction of the
plastic to air. For average glass the change in distance is
about 1/3rd the thickness of the glass and for most plastics
would be less because the index is lower than average glass.
Since the speed of light in plastic is slower than in air
(this is actually what determines the index of refraction)
the path lenght would be slightly shorter than without the
plastic.
Note that when light that is not collimated passes
through a plane block there is a variation of the bending
with the angle of incidence so the parallel plate causes
some spherical aberration. Because the index of refraction
also varies with the color (dispersion) the plate also
causes some chromatic aberration. These effects become more
pronounced as the light beam becomes more "vergent". This is
one reason a fairly thick glass filter works OK for general
photography where one is concerned with distant objects
where the light is essentially collimated but does not work
so well as a behind the lens filter or where one is doing
micro photography where the thickness of the filter becomes
a significant part of the total light path. This explains
why gelatin filters are preferred for this kind of
application. First, they are very thin, and secondly, the
index of refraction of gelatin is closer to that of air than
glass or plastic.
Note that flat, parallel plane, elements have been used
in some lenses to help correct for spherical or chromatic
aberration.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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