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