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Old February 17th 11, 11:04 PM posted to rec.photo.equipment.large-format
Richard Knoppow
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On 2/17/2011 2:32 AM, Richard Knoppow wrote:
If your Wide-Field Ektar has satisfactory contrast
and
color rendering its a good indication that the problem
with
the C-E was not the coating. The WF Ektar has the same
coating plus it has _four_ glass-air surfaces.



That's true and I do use a LOT of cameras from that time
period with single coated and even uncoated lenses so I'm
not sure what the issue was.

Another experience, I ran into the same sort of problem
with a 135mm xenar LF lens. It was a -horrible- lens where
the 75mm xenar on my rolleicord is a wonderful one. So
just because a lens has more elements in my experience
doesn't mean it's always a better performer.

Stephe

I had a chance to test a 135mm, f4.5 Xenar as supplied
on some Speed Graphics, it had a problem similar to the
Wollensak Raptar/Optar in that while the center was quite
sharp the corners never got completely sharp even when
stopped down all the way. This problem seems to be uniform
throughout the Rapar/Optar line, including the enlarging
lenses, but not for the f/5.6 lens made for the Graflex
Super-D or the Wollensak telephoto lenses, both of which are
very good. I also have Rollei cameras with f/3.5 Xenars and
they are extremely sharp with good corner performance. All I
can think is that the same prescription was not used for
other focal lengths or for other speeds. Its very odd. So, I
don't know if you got a dog or if all of the Xenars are that
bad.
More elements give the designer more resources to make
corrections. The Cooke Triplet has six surfaces, two
spacings, and three powers, just enough to correct all seven
of the primary aberrations and maintain focal length. More
complex lenses allow for correction of secondary aberrations
so they are valuable where one needs a faster or wider angle
lens. Also, by using a symmetrical design the lateral
aberrations, lateral color, coma, geometrical distortion,
are, at least to some degree automatically corrected by the
symmetry.
While the Triplet is a simple lens it is _not_ cheap to
make because the element spacing is very critical so that
the mount has to be precision made.
The Tessar was not inspired by the Triplet but the
Heliar was. Hans Harting, of Voigtlander, tried to correct
some of the secondary aberrations of the Triplet by
compounding the two end elements. His original design was
not successful but later designers did much better. The
Voigtlander Heliar (which became a different design than
Harting's original), the Dalmeyer Pentac (of Booth), and the
Kodak Ektar used on the Medalist camera, and the Kodak
Enlarging Ektar lenses of 50mm and 75mm are examples of
excellent lenses based on this general design. However,
other lens types offer more possibilities to the designer so
the Heliar type has not been popular.


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Richard Knoppow
Los Angeles, CA, USA