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Oddity with Protar VII
"Cheesehead" wrote in message ... I was looking @ the rear cell on my B&L lens, and it's 23 1/4" f/l. The front cell is 18 7/8" f/l. The old B&L table never put a longer cell in the rear. http://www.cameraeccentric.com/html/...schlomb_3.html I got the camera from the original family owner, and it was apparently ordered this way in 1923. Makes me wonder ... any thoughts on this anomaly? Either cell will fit on either end, somone 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. Both cells should have the same threads and both sides of the shutter should be the same. Both B&L and Zeiss sold Protars in sets with up to four cells of different focal lengths which could be combined or used alone. The most elaborate B&L set also came with an extra-wide-angle Protar. -- Richard Knoppow Los Angeles, CA, USA |
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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? -- .~. Jean-David Beyer Registered Linux User 85642. /V\ PGP-Key: 9A2FC99A Registered Machine 241939. /( )\ Shrewsbury, New Jersey http://counter.li.org ^^-^^ 19:55:01 up 11 days, 4:28, 3 users, load average: 4.85, 5.00, 5.04 |
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