Recycling optical elements, lens design 101

[Robin Harrison]

I'm trying to adapt a random old lens for use on my Panasonic G1 (the liveview is great for trivialising problems of focus register and stop-down metering). I have everything kind of sorted. Given the constraints of the body/lens, I knew the lens would end up focussing too close. It does. So I bought a couple of Zeiss Distar filters (opposite to close-up filters) , -1.5 and -2.5 strength. These two in combination are roughly right to achieve good focus, but the solution is a little clunky.

My question is: has anyone any experience of raiding old lenses for optical elements? How would one test the focal lenth or diopter of the elements within? Can you tell from the lens element diagrams that are sometimes available? I have a couple of old SLR teles that I suspect would have elements of a decent diameter, but I'd like to have an idea if they are about right before takign the hacksaw to the lens!

Also: is element order important? If a concave lens in front of the front element achieves good focus, would replacing it behind the back element have the same effect?

 

[Al Kaplan]

Yes, the order of the individual elements is very important, as well as which side faces the front or rear. You can end up with all kinds of problems, from field curvature to color fringing to vignetting, and then some. The tele and wide angle converter lenses for fixed lens rangefinder cameras required stopping way down for decent (but not great) results, and telextenders that fit between the lens and the camera, were best if they were "dedicated" designed for that particular lens.

What I have done with good success is adapt a 400mm f/6.3 Tele-Astranar so I can get infinity focus on a Visoflex II. This doesn't involve adding or subtracting elements. It does require shortening the lens barrel between the front and rear groups by a few mm and getting rid of the T-mount adapter so you can mount an M adapter where the T-mount threads were.

 

[Robin Harrison]

Thanks for the insight, Al. The positioning/orientation of the extra elements will of course be important now I think about it - just thinking of WHERE the rays converge or diverge...I guess this lens design business isn't just about putting a load of glass together in a random order after all!

I'm not too worried about the fidelity of the image too much, so long as something, somewhere in the frame is relatively in focus! The lens in question is a Tarcus 50mm f/0.95, and as you can imagine I plan to only ever really use it wide open.

Some further experiments have shown that all I should do is mount the Distars in front of the front element (it's going to look ugly...and lose me about a 1/3 of a stop of light) and see what results I can get.

 

[Sonnar Brian]

It depends on the effects that you want for element order. Most front and rear groups in lenses form a decent optic by themselves. Putting them in combination is likely to produce an "uncorrected" image. The distance between the groups also controls focal length and back=focus. Think of it as a zoom lens that you are fixing into position.

So far, my favorite is half of a J-8 combined with half of a Xenon, set for the Leica standard and RF coupled.

http://ziforums.com/showthread.php?t=138


Wide-Open on the Bessa R2:

Stopped Down to F4:

Distance Shot at F2: We're NOT in Kansas Anymore, Baby!

Also Stopped Down to F4:

I'm thinking the Harry Potter movie used one of these for the memory viewing sequences.