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Old October 18th 11, 01:08 AM posted to rec.photo.equipment.large-format
Jean-David Beyer
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Default Oddity with Protar VII

Richard Knoppow wrote:

Either cell will fit on either end, som[e]one just reversed
them from normal. The correction is slightly better when the
longer lens is in front. When the single cells are used
alone they perform best behind the diaphragm but there is
really little difference in correction. Since the principle
points of the single cells are displaced in the direction of
the curvature the distance from the focal plane is less when
the lenses are mounted on the front of the shutter. This
allows some lenses to be used on bellows which are not long
enough to focus them when on the back of the shutter. The
difference is significantly more than just the mechanical
length although that also counts.


I have a Wisner Convertible Plasmat set with five f/13 elements.
If you use two at a time, the aperture is somewhat larger; e.g.,
if you use both 250mm elements, the maximum aperture is f/9 and
the focal length is 152mm. If you use the 400mm and 450mm elements
together, the maximum aperture is f/9.6 and the focal length is
258mm.
The cells are two 250mm, one is 350mm, one is 400 mm, and one is 450mm.

If you use just one element, he says to place it behind the diaphragm.
If you use two, he say to put the longer lens behind and the shorter
one in front. Of course it makes no difference if you use both 250mm
elements together.

When you say the correction is slightly better with the longer element
in front, do you mean in general, or just the Protar design?

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