Quote Originally Posted by David Lyga View Post
To clarify: you CAN focus that lens through an SLR. Not for picture taking but you can focus merely to see if that lens is capable of sharpness. You hold the lens in front of the mount and look through the viewfinder. Then you slowly move the lens back and forth until you get a sharp image. This sharpness will be fleeting and transitory but it will be there. Register distance does not matter here as you are not affixing the lens to the mount, merely seeing if the lens is CAPABLE of sharpness. Yes, even with an enlarging lens you can do this. - David Lyga
The flange/film distance is of impact in such a test-set, as you won't be able to focus to infinity. You would have to try to focus at something nearby. In the worst case no focussing would be possible at all or you would end up at a scale where other abberations might show up and hamper evaluation.