[pure-silver] Re: correction for bellows extension
- From: `Richard Knoppow <dickburk@xxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Wed, 04 Mar 2015 21:52:46 -0800
On 3/4/2015 6:24 PM, Jean-David Beyer wrote:
On 03/04/2015 09:08 PM, `Richard Knoppow wrote:
My Wisner convertible Plasmat Set has the aperture dial engraved in
millimeters because there are 11 different focal lengths you can get by
using one or two of the cells. There are 5 cells.
2 are 250mm f/13
1 is 350mm f/13
1 is 400mm f/13
1 is 450mm f/13.
If you use two at a time, the aperture is greater.
2 250mm come out to f/9 with 152mm focal length.
the 400 and 450 together come out to f/9.6 with 258mm focal length.
Anyhow, there is no way Ron Wisner could have engraved all that on his
modified Copal #1 shutter.
My Zeiss Convertible Protar, that the burglar got along with my
5x7 Agfa camera, was the same way, the aperture is calibrated in
millimeters because there are three focal lengths. Complete Protar kits
had four elements so were the same way. I calibrated the stops by
measuring the entrance pupil and made a chart showing the stops for each
focal length. There are even more combinations than may be apparent at
first. While the optimum position of the single cells is behind the
shutter they can be used in front too and the longest focal length must
be because the bellows draw is not long enough to focus them at
infinity. In back of the shutter the pupil is the aperture stop so but
when used in front the lens magnifies the stop so the f/stop is larger
than the physical size of the hole and also its optical position is
changed. These cells are meniscus lenses as are the single cells of a
Dagor. Their principle points are shifted toward the convex end of the
lens so when used behind the shutter they are slightly retrofocus and
when in front slightly telephoto. As a result they need more bellows
draw when used in the optimum position not only because the lens as a
whole is closer to the film but because the principle point on the
convex end is slightly displaced toward the film. The reason for using
the lens on the short light path (I am drawing a blank on the correct
name for this) is that when the iris is on the long side it tends to
correct field curvature and reduce some other aberrations. However
turning the "wrong" way can have some benefits so meniscus lenses on box
cameras were nearly all facing outward.
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
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