[rollei_list] Re: Bright Screens
- From: Kirk Thompson <thompsonkirk@xxxxxxxxxxx>
- To: Rollei List <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 4 Jan 2012 07:19:00 -0800
Thank you Richard for this thorough explanation of pitfalls in the quest for
sharpness.
Focus shift is even more a problem now that digital sensors (perfect 'film
flatness') reveal small lens disparities. Leica recently re-designed the 35mm
Summilux because it had earned a bad rap for focus shift. Sean Reid, on the
best website for review of lenses for Leica, now tests all lenses for focus
shift. Several of the Cosina Voigtlander lenses in Leica mount have done
poorly in this respect.
One hears that Tessars can resolve as well as Planars if stopped down to f8.
No doubt this is where DOF overcomes Tessar focus shift.
Fortunately for photographers, absolute sharpness isn't the only value in a
lens. Other aspects of how a lens 'draws' or renders can be more important
than how well it reveals pores & beard hairs.
Kirk
> From: dickburk@xxxxxxxxxxxxx
> To: rollei_list@xxxxxxxxxxxxx
> Subject: [rollei_list] Re: Bright Screens
> 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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