[rollei_list] Re: Bright Screens
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 3 Jan 2012 22:55:32 -0800
----- Original Message -----
From: "Kirk Thompson" <thompsonkirk@xxxxxxxxxxx>
To: "Rollei List" <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, January 03, 2012 9:17 PM
Subject: [rollei_list] Re: Bright Screens
Maxwell screens come with DIY instructions, but a surprising
number of people put screens in upside down – two of my
Rolleis came that way.
I've always had Krikor or Mark Hansen install the screens
and colimate the lenses at the same time. Mark says that
the majority of TLRs he works on need this (he does Rollei &
Zeiss).
IMO lens calibration should be done by a tech who has a
colimator & doesn't just eyeball the focus on a piece of
ground glass. But you could do the latter yourself.
Krikor's Rollei colimator sets lenses so 60' = infinity;
Mark, Infinity = infinity. I've heard that some techs put a
piece of film in the camera for 'average curl,' while some
use a piece of mirror, for absolute flatness. Apparently
there are diffferent approaches to this.
Kirk
Its worse that that! Most lenses have enough residual
spherical aberration to have some focus shift. Focus shift
is where the point of apparent sharpest focus changes as the
lens is stopped down. The amount varies with different
designs; Dagors have a lot, Planar derivatives have very
little. The Tessar type lens, common on Rolleis, has enough
to be detectable when going from wide open to perhaps two
stops down, after which its pretty much gone. Even minimal
focus shift can cause problems in cameras that are focused
indirectly, that is by looking at some focus indication than
a direct image from the lens at the _working_ stop. This
applies to cameras using rangefinders, twin-lens reflex
cameras, and single-lens reflex cameras where the focusing
is done with the lens wide open. While lack of film flatness
is often blamed for focus errors, and this might be right,
focus shift is also to blame for some. There is no fix for
this other than to set the focus at some stop smaller than
wide open. Of course, that just changes the stop that
exhibits the largest error. Depth of focus will compensate
to some degree for this fault so usually focus is set with
the lens wide open with the assumption that any error will
be masked by the increase in depth of focus at smaller
stops. Lenses of the Planar/Biotar type have, in general,
small focus shift which is another advantage they possess
when one needs a faster lens.
Film flatness can probably be measured with an
interferometer. Such an instrument can record the position
and contour of the film plane. I've not seen this done in
practice but rather think it has been for applications
requiring very accurate focal plane location such as in
mapping cameras.
FWIW, the autocollimator provided by Rollei had a first
surface mirror to be installed in the film gate. Of course,
one can choose any distance for the finder to correspond
with infinity for the taking lens.
I have doubts about whether a camera repair person
could determine the curl of film without rather complex
instrumentation as suggested above. Certainly any result
gotten visually with the back plate removed would be
invalid.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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