View Full Version : Changing element spacing on raptar?

C. D. Keth
15-Nov-2006, 00:02
So I have a Wollensak enlarging raptar with 190mm focal length. I've tried it as a taking lens and while I kind of like it, I have a similar length lens I like better.

Can anybody help me predict what would happen if I spaced the elements differently? The best info I can find tells me that the raptar design is a rear positive cemented group (of ~120mm focal length) and a front negative air-spaced group (which I don't know how to measure the focal length). The iris appears to be right in the middle of the two.

The faces of the groups that face each other look to be about 5mm apart when they are all mounted up. What would happen if I mounted them farther apart, like 2cm?

Donald Qualls
15-Nov-2006, 14:39
The layout you describe is similar to that of a Tessar. My experience with changing spacing between front and rear cells with Tessar type lenses rapidly introduces aberration with little change in focal length. Changing the spacing between front and second element, OTOH (keeping the second the same position relative to the rear cell, moving the front element forward -- usually possible with some combination of shims) adjusts focal length without an undue increase in spheric aberration -- this is how front-element focusing lenses are adjusted, and many of those are quite good.

If you want to shorten the focus of the lens you have, you could try making a pair of matching shims, one to go between the front cell and shutter, and the other between the second element's inner cell and the main front cell (both should have shoulders that will accomodate shims). The total travel required is only going to be 2-3 millimeters, and the more you change the spacing the more spheric aberration you'll introduce; typically, however, you can shorten the focus enough to be equivalent of adding a +1 diopter (equivalent to focusing the lens to one meter without moving the lens board) without excessive deterioration.

Jim Galli
15-Nov-2006, 16:33
Yes, the Raptar is a common Tessar. It's old tired uncle was the Velostigmat which Wolly used to make so that you could put seperation between the 2 air space elements just for the purpose you intend. Diffusion. Basically it altered the focal length of the lens slightly throwing everything slightly out of focus. Then if you re-focused in the diffused position, it was sharp again. In other words you could accomplish the same thing by moving the focus plane so everything is blurry.