Something I mentioned a while ago was attempting to fit an old large format lens onto a Holga 120 Pan with the aim of ending up with a 6x12cm shooting camera that takes reasonable quality photos on the cheap. (£100'ish in parts vs £1000 for a proper 6x12 camera) There's a few guides on the internet but the most comprehensive one I found was this PDF which I pretty much followed to the letter, apart from one key point which turned out to be a rather major stumbling block.
BUILD
FOCUSING PROBLEMS
So the one major difference was that instead of the recommended Schneider Angulon 90mm lens I ended up using a much older Graflex Optar 90mm from the 1950s, mainly for reasons of cost, I'm not sure if the Angulons are extra popular at the minute but the cheapest I could find was in the £150 range where as the Graflex cost me £50. I assumed this wouldn't cause any problems, they are both 90mm large format lens so the same guide should apply right? Unfortunately this wasn't quite the case and I've had real problems trying to get the camera to focus properly without closing the lens down to f22.
I don't know if the flange focal distance / optical centre is different between the Angulon and Graflex lens but from what I can figure out the 12-17mm focusing helicoid along with the M42 body cap means that the lens sits 2-3mm too far forward, resulting in the lens not being able to hit infinity focus. I swapped out the 12-17mm helicoid for a fixed 7mm macro tube, and this had the opposite effect of moving the lens too close to the film plane. So I'm either back focusing or forward focusing and the only way to get a sharp image is to stop the lens right down.
In the images below you can see that with the 7mm macro tube the focus is off in the distance and the in-focus area moves forward as the lens is stopped down, where as with the helicoid at 12mm the focus starts off up close and expands off into the distance as the lens is stopped down, neither of which is ideal.
7mm Macro Tube @ f6.8/f11/f16/f22/f32
12mm Helicoid @ f6.8/f11/f16/f22/f32
The two options are to try and extend the 7mm macro tube by a few more mm and have it as a fixed focus camera, or try and move the 12-17mm helicoid back a few mm which would mean chewing up the front of the camera. I'm undecided which way I'll go but its been a great learning experience, reading up on focal lengths and finally understanding how lens focus, that and its always fun cutting stuff up and trying to put it back together.
Thanks to Niamh for milling out the middle of the M42 body cap on the lathe, I blatantly would have made a right mess of it or lost a finger in the attempt.