Hi (not sure if this is the correct place to post this, please move the post to the appropriate location if it isn't). - I know Brian has explained how to reduce the focal length of the Jupiter-3's and 8's in various forums in the past, and I have had the pleasure of using his informative answers in my own adjustment (shimming) cleaning and lubrication-jobs on my Jupiter-8's. Though, one of my lenses seem to be "long in the tooth", that is, on the long end of the nominal focal length. It's spot on close up and increasingly front focusing the further away you get form the target at F2. - Only a stopping down to around F8 will make it ok from 5 meters and further away. So I am trying to reduce the focal length, however. The question I have is this: Before removing the rear lens group, I always put some wallpaper-tape (paper tape) which covers the whole threaded area of the optics fixture + the rear optics group. Then i cut the tape with a knife, so that I am able to screw out the rear optics group. The optics group then has a small piece of the tape and the rest is left on the optics fixture (I find this better than marking the alignment with a sharpie). When I then start sanding the rear part of the optics fixture, I can clean the area regularly and screw the rear lens group back on and observe how much the tape-pieces have moved to each other. Two questions: - I've seen the formulas on calculating the focal lengths, but I don't have the equipment necessary to perform anything else than "put a matte-screen where the film is and check focus with a magnifier". How to you measure a lens actual focal length..? I do own a pair of calipers, and even though it actually, digitally, can measure down to 0.01mm, it's not practically possible to measure up the lens with it. (each measurement gives different figures). - When I sand (I use wet-paper to sand, which creates much less metal filing-mess) I have sanded so much that I know that the rear group now screws one additional round (360 degrees) before it stops, compared to when I first put the tape on. So the rear group is "one round closer" to the aperture and front lens group. Still, the actual focal length _distance_ reduction in mm is hard to tell. Does anyone know how much actual distance closer the rear group moves in a 360 degree turn? After sanding so the rear group now comes "one turn closer" to the aperture blades, i also needed to sand the original shim down from 1.4 mm to 1.10 mm, so the lens is spot on at close focusing limit. But I still see that the lens is still horrible front focusing, when comparing to previous test-shots, it seems that the focus error has hardly moved at all. So I really don't know if I have sanded to little, or if the lens is faulty somehow. - Since I really don't know how much I have reduced the actual focal length with, just "one turn" of the rear group. The distance-scale on the lens itself seem to agree pretty well with the 1 meter, 5 meter 10 meter shoot distances. I can see if I am able to attach some actual photo-tests a little later. Tips?