Finding focal length of achromat

[IanPhoto]

I took the lens and shutter off a Bilora Bella 44, a camera for 127 film. I then took the guts out of a Kodak Tourist and replaced it with a piece of wood on which to mount the Bilora Lens/shutter. Problem is, when I took the front element off the Bilora lens set for cleaning, I didn't mark the relationship between the 2 lens elements at the factory-set infinity position. Now I don't know how close the two lens elements are supposed to be at infinity. I used a ground glass to try to set the flange-focal distance but I am confused that if I don't have the right element spacing then I will come up with a non-optimal flange-focal distance. I think if I screw the front element in as far as it will let me then create the flange focal distance from that (using ground glass technique), I can then unscrew the front element from that point and if it gets any clearer then that is my infinity mark but I don't believe this is the "correct" way. Should I do some sort of collimation test with just the lens and then solve for flange focal distance in the camera??

 

[Denverdad]

Can the lens assembly still be mounted back onto the Bilora at this point? If so it would probably be easiest to go ahead and do that first and then work on re-setting the lens spacing while in that configuration. The reason is that the lens's interface to the original body defines the correct back focal spacing for you, so it basically eliminates that as a variable in the process. If you can do this then we can probably make some further suggestions for how to proceed.

I doubt you will be able to set optimal lens spacing with confidence by using any sort of collimation test. "Correct" spacing is all about minimizing aberrations at certain conjugate distances, and that is something I think would be very difficult to judge just by visual inspection of images on a ground glass. Then again, I don't know how critical the lens spacing is in the first place - it could be that for a simple achromat just getting the focusing element within one or two turns may be good enough.