Re: Question on "normal" lens

[P3D Eric Goldstein <egoldste@xxxxxx>]

Dr. George A. Themelis ranted as in the old days:

> The fact that the FL of the lenses in the Belplasca matches the diagonal
> of the film frame to a fraction of a mm, means absolutely nothing to
> anyone, except mathematicians and so called "purists" of the wrong type!
> (now, is that the good old DrT, or what??? ;))

Good show, George! It's good to see that you haven't lost your touch. 8-)

> Seriously, the definition of the normal lens is a convention, not an
> exact science.  It means little in practical terms.

I probably should be speaking for him, but I think Dennis' point, in fact, 
the point we were both making, was that a 35 mm lens of tessar design will 
have problems covering the width of a 7 perf format. Even if illumination is 
ok, resolution will fall off in the corners. Leica made a 35 mm f/3.5 Elmar 
(same as tessar) for 8 perf and it was not a sharp lens; they went to a 6 
element design (Summeron I think) as did Rodenstock and others. When Rollie 
introduced the Zeiss Planar lens on their TLRs in the mid-1950s, they 
increased the focal length of the faster 2.8 versions to more adequately 
cover the corners. This is a common practice in lens design, and it is in 
this realm that the diagonal mesure of the format does translate into 
practical design considerations.

By using a focal length a bit closer to "normal," Belca was able to get 
excellent resolution at the edges on the Belplasca. In fact, one lens test I 
did had center *and*corner*resolution* virtually equal at sixty-some odd 
lppmm at f/8 or f/11... not all that many modern comsumer prime lenses will 
perform to this level, and very few modern consumer zooms even come close! 
RBT, eat your heart out! (Just kidding Jon)

The 35 mm lenses on a Realist exhibit a fall off in resolution in the 
corners of a 5 perf frame; the fall off at 7 perf should be quite dramatic. 
But depending upon what you shoot, you could use this corner softness to 
your creative advantage...You guys could market it as the "stereo softar!" 
8-)

Eric G.


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