[pure-silver] Re: Swapping Out Lens Elements On Ektars
- From: "Nicholas O. Lindan" <nolindan@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 20 Mar 2014 00:23:51 -0400
"Richard Knoppow" wrote:'
Note that Tessars of all makes suffer from oblique spherical
aberration. ... A good Tessar should be pretty much free of it
at about 25degrees half-angle, when stopped down about a stop
and a half. For an f/4.5 lens it should be comletely gone at
about f/11 at the corners of a 4x5
The Zeiss branded Tessar on the Yashica T4/T5 is a 35mm f3.5
and is sharp in the corners wide open. The camera doesn't
start to stop the lens down until the shutter speed is 1/200
or faster (or something in that neighbourhoud).
This sort of performance isn't usual for a Tessar - I would
assume the T4's Tessar is a reformulated lens based on the
classic tessar.
As to spacing - front element focusing is common on triplets,
but I have never seen front group focusing. A Tessar can be
looked at as a triplet with an achromat rear element - although
it
is said it was derived by a different path from a double
achromat.
The Schneider Angulons (the inside-out Dagor design) that I
have are shimmed to compensate for either variations in shutter
thickness and/or variations in the lens elements.
I would think that measuring the distance with a vernier caliper
to insure the spacing is the same would be more than adequate.
Shims can be made from paper.
--
Nicholas O. Lindan
Cleveland Engineering Design, LLC
Cleveland, Ohio 44121
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