M39, L39, LTM, Visoflex L39, Zenit M39, Canon Screw Mount... Confused? So are most people. It doesnt help that there are 6 different mounts all 39mm in diameter, 3 different thread pitches, 4 different focal flange distances and different jargon for the mounts depending on who you talk to!
|L39||39mm||0.977mm||26||28.8mm||LTM (Leica Thread Mount)|
|Visoflex L39||39mm||0.977mm||26||91.3mm||Leitz Visoflex I|
|J-Mount||39mm||1.06mm||24||28.8mm||Canon Screw Mount|
True Leica Thread-Mount (LTM) is 39 mm in diameter and has a thread of 26 turns-per-inch or threads-per-inch (tpi) (approximately 0.977 mm pitch) of Whitworth thread form. Whitworth threads were then the norm in microscope manufacture. The Royal Microscopical Society (RMS) thread, also known as society thread, is a special 0.8" diameter x 36 tpi Whitworth thread used for microscope objective lenses and Leitz was a major manufacturer of microscopes, so the tooling at the plant was already set up to produce the Whitworth thread form. The Soviets in the 1930s produced their early FED cameras in M39×1 (39 mm by 1 mm DIN thread). Early Canon cameras also used a different M39 × 24 tpi thread mount, called "J-mount".
True LTM lenses have a flange focal distance of 28.8 mm, though this is of little importance for lenses used on bellows enlargers. The Soviets later adopted the LTM mount for their Zenit single-lens-reflex (SLR) cameras, though with the longer optical registration of 45.2 mm, required to allow the mirror room to flip out of the focal path when a picture was taken. Until the 1970s the 39 mm mount was the norm for exchangeable lenses in rangefinder cameras. The high cost of quality lenses led to the dual use camera/enlarger of the lenses, hence the fact that enlargers also accept 39 mm lenses.
Wikipedia: M39 Lens Mount (https://en.wikipedia.org/wiki/M39_lens_mount)
TL:DR - There are lots of different variants of the 39mm Screw Lens Mount and it can be hard to find out which mount, and adapter, is best for your needs. We've found that the smaller pitch lenses, like L39, will mount on the larger pitch adapters, like M39. But the reverse is not necessarily true, sometimes it works, sometimes it won't. And even if it does fit, depending on the focal flange distance of your lens based on the chart above, it may not focus properly. The best option usually is to try an adapter out, and if it doesn't work keep in mind that Fotodiox has a 14-day return policy so you have 2 weeks to try it out and make sure (but you'll know within a couple seconds if the lens will fit and focus or not).