Donald Qualls
17-May-2005, 09:47
I recently acquired a 1930s vintage 13.5 cm f/4.5 Skopar, glass in excellent condition (though the filter thread is pretty banged up -- doesn't matter much, since I own only one filter and no suitable step-up ring), intending to use it to replace the Radionar in my ca. 1935 Kawee Camera (9x12 cm plate camera). I was dismayed to find, however, that the Skopar has about 3 mm shorter focal length and as a result I can't obtain infinity focus when it's mounted.
I've examined various ways I might be able to modify the camera (preferably in a reversible fashion -- I'd like to retain the option to reinstall the original Radionar and sell the camera in original configuration if I choose at some future time), and come to the conclusion there isn't a good way. The front standard latch must be against a stop in order to keep the standard parallel to the film plane, even if it were practical to remove the distance scale and infinity stop and drill/tap new holes to mount it further back. Any method I've examined for shimming the standard would require elongating its latching point enough that it wouldn't latch properly with the shim removed (which would then require shimming the shutter if the original lens were reinstalled).
Then it occurred to me I might be able to modify the lens (or, correctly, the lens cell).
The Skopar is a "copy" of the Tessar -- possibly not identical prescription, but optically similar in terms of type, strength, and glass type of each element: positive meniscus front element, biconcave middle, and cemented doublet, net positive, rear group, with stop and shutter between middle and rear. Though I don't know of Voigtlander ever making such a version, Carl Zeiss Tessars were commonly made for front element focusing, starting around the time this Skopar was produced; effectively, increasing the spacing between the front and middle elements shortens the focal length of the lens assembly.
Thinking about that, it occurred to me that it might be possible to shorten the spacing in the Skopar (by about half a gnat's whisker, given that 1 mm with a 10.5 cm Tessar is enough to change focus from infinity to around 10 m) in order to increase the focal length by the critical 3 mm I need to make it work on my Kawee (and a tiny overshoot would be acceptable; I can shim the shutter or the elements if the focal length goes too long; there's no way, however, to install minus 3 mm of shims behind the shutter). Question is, how much will I degrade the optical quality by doing this? I know front-focusing Tessars are widely considered optically inferior to otherwise identical unit-focusing Skopars (in medium format) because the variation in spacing reduces image quality -- but by how much? Is it something I might notice in making, say, 16x20 enlargements from the 9x12 cm negatives (just over 4x), or would I have to be cropping and enlarging by much larger factors to see the effects? How far can this go and still keep the Skopar ahead of the Radionar for overall image quality (it covers a good bit more, too, so if the image is the same at, say, f/16, the Skopar still wins; I've gotten fuzzy edges a couple times from using relatively small amounts of front rise with the Radionar).
Modification would be done with very fine sandpaper on a flat surface, removing a tiny bit of material from the rear shoulder of the front element cell, against which the middle element cell mates, in order to allow the middle element to enter more deeply (by a fraction of a millimeter); the procedure would be to sand a little, clean and reassemble the lens, and check for infinity focus, then repeat as necessary. There's no danger of sanding into the glass from this direction, as it's at the front and I'm sanding the rear.
Bottom line: is this something with a reasonable chance of success, or should I instead look to trade or sell this glass (which came to me in a thoroughly dead, i.e. partly disassembled and probably missing parts, rim-set #0 Compur, 250 top speed) and keep searching for a Tessar type classic lens that will have the focal length I need?
I've examined various ways I might be able to modify the camera (preferably in a reversible fashion -- I'd like to retain the option to reinstall the original Radionar and sell the camera in original configuration if I choose at some future time), and come to the conclusion there isn't a good way. The front standard latch must be against a stop in order to keep the standard parallel to the film plane, even if it were practical to remove the distance scale and infinity stop and drill/tap new holes to mount it further back. Any method I've examined for shimming the standard would require elongating its latching point enough that it wouldn't latch properly with the shim removed (which would then require shimming the shutter if the original lens were reinstalled).
Then it occurred to me I might be able to modify the lens (or, correctly, the lens cell).
The Skopar is a "copy" of the Tessar -- possibly not identical prescription, but optically similar in terms of type, strength, and glass type of each element: positive meniscus front element, biconcave middle, and cemented doublet, net positive, rear group, with stop and shutter between middle and rear. Though I don't know of Voigtlander ever making such a version, Carl Zeiss Tessars were commonly made for front element focusing, starting around the time this Skopar was produced; effectively, increasing the spacing between the front and middle elements shortens the focal length of the lens assembly.
Thinking about that, it occurred to me that it might be possible to shorten the spacing in the Skopar (by about half a gnat's whisker, given that 1 mm with a 10.5 cm Tessar is enough to change focus from infinity to around 10 m) in order to increase the focal length by the critical 3 mm I need to make it work on my Kawee (and a tiny overshoot would be acceptable; I can shim the shutter or the elements if the focal length goes too long; there's no way, however, to install minus 3 mm of shims behind the shutter). Question is, how much will I degrade the optical quality by doing this? I know front-focusing Tessars are widely considered optically inferior to otherwise identical unit-focusing Skopars (in medium format) because the variation in spacing reduces image quality -- but by how much? Is it something I might notice in making, say, 16x20 enlargements from the 9x12 cm negatives (just over 4x), or would I have to be cropping and enlarging by much larger factors to see the effects? How far can this go and still keep the Skopar ahead of the Radionar for overall image quality (it covers a good bit more, too, so if the image is the same at, say, f/16, the Skopar still wins; I've gotten fuzzy edges a couple times from using relatively small amounts of front rise with the Radionar).
Modification would be done with very fine sandpaper on a flat surface, removing a tiny bit of material from the rear shoulder of the front element cell, against which the middle element cell mates, in order to allow the middle element to enter more deeply (by a fraction of a millimeter); the procedure would be to sand a little, clean and reassemble the lens, and check for infinity focus, then repeat as necessary. There's no danger of sanding into the glass from this direction, as it's at the front and I'm sanding the rear.
Bottom line: is this something with a reasonable chance of success, or should I instead look to trade or sell this glass (which came to me in a thoroughly dead, i.e. partly disassembled and probably missing parts, rim-set #0 Compur, 250 top speed) and keep searching for a Tessar type classic lens that will have the focal length I need?