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 |
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 |
Oddity with Protar VII
"Cheesehead" wrote in message ... On Oct 16, 1:48 pm, "Richard Knoppow" wrote: "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 Thanks. I've also gotten the impression that the Protar is the same optical formula as the old casket lens sets. There were a number of lenses available in "casket" sets, both Zeiss and Bausch & Lomb sold such sets but other variations on the design were also available. Rudolf Kingslake shows five variations of five element cemented lenses in his book on lens history. All have similar performance. The Turner-Reich lens, which was probably designed by Ernst Gundlach is a five element cemented lens very similar to the Series 7 Protar but with one of the elements split, perhaps to get around the Zeiss patent. The T-R lens was also available in sets. Its performance is not as good as the Zeiss or B&L protar but lots of triple convertible T-R lenses were built on military contract during WW-2. The advantage of the Protar Series VII over the Dagor is that the individual cells are corrected for coma so they are sharper over a considerably wider angle than a single Dagor cell. The Dagor as a combined lens is free of coma because of the symmetry. A combination of Protar cells is further corrected for coma by the symmetry or near symmetry even when cells of different focal length are combined. While none of the single cells is as good a lens as a complete lens they were good enough when work was done on large format which was not enlarged or at not enlarged much. They were an economy where a commercial photographer wanted to have the resources to do a variety of work. B&L included an extra-wide-angle Protar in their largest combination sets with either a Volute shutter or an adaptor plat so that it could be used on the Compound shutter usually supplied with the set, as a front shutter. There are some old Zeiss and B&L catalogues at http://www.cameraeccentric.com which show the variety of protar lenses sold by both Zeiss and B&L and, I think, also a Gundlach catalogue or two. Ernst Gundlach discovered the method of splitting elements of existing designes as a way of bypassing patents pretty early. I don't think much of Gundlach lenses but the company also made Korona cameras which were pretty good. -- Richard Knoppow Los Angeles WB6KBL |
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