Re: My 7p Realist!!!

[P3D emdesher <emdesher@xxxxxxxxxxxxxxxxxxxx>]

On Fri, 10 Jan 1997, P3D Eric Goldstein wrote:

> Dr. George A. Themelis wrote:
> 
> > I contacted John
> > and asked him about converting a 2.8 to 7p.  The 4-element lenses of the
> > 2.8 could (at least in theory) do the job as well as those in the
> > Belplasca.
> 
> 
> Except a Tessar ain't a Tessar ain't a Tessar!
> 
> One tessar design is not necessarily the equal of others. Different
> element design and placement as well as different glass types and
> coatings all factor in. In terms of the lenses found in stereo cameras,
> the David White tessar type lenses perform differently from the
> "Germany" lenses, which are different from the Ektars, and the Zeiss
> belplasca lenses seems to best them all having been recalculated from
> prewar designs to take advantage of so-called rare earth glass and
> proprietary coatings (though I have one source which also claims this
> may also be true for the Ektar lenses).
> 
> In terms of coverage, the belplasca's zeiss tessars have 37.5 mm lenses
> instead of the usual 35 mm (or 36 mm for Ektars). This 7ish% increase in
> focal length may very well have been deployed to gain better coverage of
> the 7 perf format.
> 
> 
> Eric G.
> 
> 
This is a very valid comment concerning the Carl Zeiss lenses, and the 
37mm focal length was a deliberate design choice.  If you measure the 
diagonal of the 7-perf frame, it's exactly 37mm-thus it's the "normal" 
length. Like a 50 or 52mm lens on a 35mm SLR.  Also, I am supposing the 
perceived superior lens quality is also due to contrast being maximized, 
along with lens resolution.  One reason the same optical design, a Tessor,
could have different results depends on the degree of correction 
(pre-computor era, a very math intensive project) for optical light rays, 
and just as important, the quality control maintained during manufacturing.
Dennis


------------------------------