At 8:13 PM +0000 12/27/01, olympus-digest wrote:
>
>Date: Thu, 27 Dec 2001 11:34:01 -0800
>From: Scott Gomez <scott@xxxxxxxxxxxxxxxxxxxxxxxxxx>
>Subject: [OM] Eyecup and diopter correction
>
>I could use a little help here. As I mentioned in a message yesterday, one
>of the results of the Christmas photo exchange was that my girlfriend has
>conned me out of an OM-4. The problem that results is this:
>
>She can't get enough correction via the built-in adjustable viewfinder
>correction. She says she needs about a -4.5 correction. The viewfinder range
>is +1 to -3 according to the OM-4 Instructions Manual. The E-SIF lists the
>corrective range of the lenses for the eyecup as +2 to -5 *in conjunction*
>with the camera's own built in "bias". Based on what I read in the E-SIF
>tables, the viewfinder is a default -2 diopters (I'm assuming that's for
>NON-adjustable viewfinders).
>
>I think I need to buy her a "-5" lens (as Olympus marks them), as that
>coupled with the viewfinder's built-in adjustment should give her enough
>range to adjust, but I'm not sure. Does anyone know how to figure the proper
>correction, based on adding one of the eyecups with a corrective lens? Can I
>use info from her eyeglasses prescription to figure it out?
If yor girlfriend says she needs -4.5 diopters correction, she's probably
quoting from her most recent eyeglass prescription for her favored eye. You
can check the prescription to be sure. In the following, we will assume that
-4.5 diopters is correct.
Lens powers expressed in diopters have a very useful property: The power of a
compound lens is the algebraic sum of the powers of the individual lenses
making up the compound lens (assuming the lenses are close together). So, to
put -4.5 in the center of the built-in adjustment range (-3 to +1), or -1, we
need to solve the algebraic equation -1 + x = -4.5, which yields x= -3.5
diopters. So, you need a -3 or a -4 lens, assuming that the lenses are not
available in half-diopter steps. In practice, great precision is not required,
and experimentation may be useful.
If your girlfriend also has severe astigmatism, a custom lens may be necessary.
For this, one has a lens made up by an optician, starting with the eyeglass
prescription, a sample viewfinder lens (for the physical size), and a copy of
the relevant part of the OM-4 manual (giving the -3 to +1 range, etc). The
optician will want to verify these. The lens must be somehow marked to allow
correct later re-installation into the finder.
Joe Gwinn
< 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 >
|