Hi willdmo,
I suggest you read my
reply from earlier, all Information about this issue has already been said there.
Originally posted by willdmo the flange focal distance between "Zenit" 39mm and M42 is almost the same (Zenit is 0.04mm shorter or so)
Do you have a source for that assertion? All sources I found stated that M39 SLR is at 45.2mm and M42 is at 45.5mm, like I said before. The difference between 45.2 and 45.5 is like I already said
0.3, not 0.04, which is not that small a difference for flange focal distances.
Here is the manual of the Zenit 3m as a source for the 45.2mm (page 4, "Working distance").
M42 is a well known standard, but for example
here is it's Wikipedia page as source for the 45.5mm.
I do not know how you calculated 0.04mm, but I dare to say that it's just incorrect.
(That's what I meant when I said
Originally posted by Aejap I hope that this will counter some of the misinformation that circulates on here and around the internet
by the way)
Originally posted by willdmo adapters made to specs work
Like I said in my mentioned reply from earlier, they do
almost work. True infinity obviously can't be reached because the M39 SLR lens has to be closer to the film plane of the M42 camera than physically possible (no mechanical adapter can change that, there would have to be a correction lens inside the adapter). No adapter ring out there will make the lens screw 0.3mm further into the camera than possible.
Of course, that might not be that big of a deal for you, as it depends on what you want to do.