A slightly different lens spacing question

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?

Donald, pardon my ignorance. It relocating the stop impossible? Failing that, is it impossible to make a spacer that will sit between the stop and the front standard?

Cheers,

Dan

Donald. OK so you would need something like a 3 mm recessed lens board because you cannot make the camera extension shorter than it is ?

Too bad that your skopar does not exhibit a lightly longer focal length... a spacer ring or plate would do the job. Personnally I would not disassemble this lens but I have absolutely no quantitative idea about go/no-go in terms of optical quality degradation when un-screwing the front lens element !!
And what about the crazy idea to re-fabricate do-it yourself plate holders, 3 mm less deep than the originals ? ;-);-)

Can't you just drill a recess in the lensboard? It's only 3mm. Use an over sized forstner bit for the recess and then use the right size bit to drill the complete hole.

All assuming the camera has lensboards.

Hi there,

Have you tried the cells in the original shutter to see if it has an effect on the back focus length?

It may just be easier to find another lens. Even if shaving the cell works, I don't think it will get you pass the designed focal length and probably will not change the back focus length.

Just a thought

I wish it were as simple as recessing the lens board, but please understand, this is not only a plate camera (no lens board, the lens mounts directly into the front standard), but the design also known as "Patent Etui" which uses a distinctly different (and patented, as of about 1926) front standard design to fold up into a more compact package than any other large format camera before or since -- when folded, this entire 9x12 cm camera is about the same size as an ANSI 4x5 film holder.

Part of the front standard design is that there's a bar across the front, and legs that form triangle braces to keep the standard rigid on its spring tensioned hinge -- but that bar has to be against a stop, and the standard "sled" held by a latch, to complete the rigid setup. Shimming the bar (possible, more or less) would require also extending the slot the latch seats in by the same amount, and not only is there limited metal available there, but that would then make it impossible for the camera to latch up correctly with the shim removed and original lens back in place. The "best" method I've found to date, short of the lens modification I was asking about, has been to grind/file the rear end of the focus rack to gain additional rearward movement, and reinstall the focus scale and infinity stop at the new location (though drilling and tapping matching holes in the bed looks like entirely too much fun). I've been resisting this because I'd like to keep, insofar as it's possible, the option to return the camera to original configuration.

I haven't yet tried installing the dead shutter on the camera and checking focus, but I have no reason to believe it will be different -- the dead one is identical model to the one I'm now using with this camera, on the Radionar.

Paul, I don't actually want to go "past" the design focal length -- rather, I'm interested in changing where the lens falls within the manufacturing tolerance. At present, this Skopar is near the short end of the tolerance; I want to lengthen the focal length to bring it near the middle (though the longer end would still be acceptable). I just don't have any idea of the magnitude of aberrations I might introduce by changing the front cell spacing on a Tessar type lens that wasn't designed for front element focusing -- hence my question.

I have a vague memory of reading somewhere that Tessars are rather sensitive to cell spacing, but on the other hand, there were plenty of front cell focussing models for the amatuer market so it will work to some extent.

Instead of shaving a small amount off the lens cells, I would shave a small amount off the shutter. Usually there is a lip of some sort that butts up against the lens cell, and gently sanding that down would be a lot less nerve-wracking than abrading the lens cells. You can even practice on the dud shutter first.

Struan, the problem with shaving the shutter is it's changing the spacing in the wrong place. Front-focusing Tessars have the middle element fixed to the shutter, and move only the front glass. If I shave the shutter, I'm (in effect) moving the *rear* glass, which might have a completely different effect. The only way I have to bring the front two elements together is to alter the front cell -- and in fact that's not completely analogous, because I'd be moving the middle cell away from the back doublet while it gets closer to the front cell, instead of keeping the spacing across the shutter constant while moving the front cell. Ideally, to emulate a front-focusing Tessar, I'd want/need to shave the shutter *and* the back of the front cell by the same amount, so as to move the front glass toward the shutter while keeping the middle element fixed in position. However, I'm not very confident of being able to remove the same amount from different metals (aluminum and brass)...

Ah well.

Unless you just like working in the machine shop, it's perhaps time to get another cheap old lens.

Hi Donald,

Thanks for the thread, I just CLA'd my K.W. 'patent Etui' 135/4.5 Zeiss Tessar. On mine the front latch has more than 3mm adjustment. There is no need to use a spacer for the stiff legs, the bed struts are adjustable to bring the front standard back to parallel by raising or dropping the bed.

Voigtlander did make a front focus lens, 'Vaskar', but I have not seen a 135mm. Your Skopar cells may be in the wrong shutter, they were not standard sized back then. K.W. did use Heliars on patent Etui cameras so they were working with Voigtlander but I have not seen one with a Skopar.

Good luck with it.