[pure-silver] Re: Uncoated lenses
----- Original Message ----- From: mmagid3005@xxxxxxx 
To: pure-silver@xxxxxxxxxxxxx
Sent: Tuesday, December 30, 2008 8:26 AM
Subject: [pure-silver] Uncoated lenses
I want to use an old f/6.3 uncoated Tessar from a 9 X 14 ICA folding camera. Is there a filter I can use that will simulate the effect of a coated lens? 


Marty
BTW, While the f/6.3 Tessar was probably an economy model Tessars work very well at this speed. One of the inherent problems of the Tessar type is an aberration called oblique spherical aberration. This looks something like coma since it results in a smearing of points in the image toward the periphery and is proportional to the stop. All Tessars have this fault to some degree but, since it is proportional to the stop (reduced by stopping down) it tends to be worse in faster lenses. f/4.5 Tessars generally must be stopped down to around f/8 to f/11 to get rid of it. There should be less in the slower lens. The slower lenses are also smaller and lighter than faster ones. Check the front cell of this lens for a cap on the back. Generally, Zeiss used caps on the backs of longer focal length lenses and retaining rings on the front of those with shorter FLs. The caps are easily removed for cleaning the inside surfaces of the elements. Where a retaining ring is used a friction wrench must be used for removal. This is simply a tube the right diameter to fit the ring with a ring of sticky rubber on one end. The design of the cells is such that elements are automatically centered by the cap or ring. These touch the surface of the element along a ring shaped knife edge. A spherical surface will always be brought to the center of curvature by such an arrangement. 

--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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