[rollei_list] Re: Bright Screens


----- 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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