Leica Leica screw mount and other screw mount lenses

Country Parson

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North Carolina
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I am confused by the designations for screw mount lenses. I bought by mistake an adapter that says M42. But when I put my calipers into the threads it reads just shy of 39mm. Is that not the size for Leica screw mounts? If so why does it say M42 and is being sold as a Leica adapter? Anybody understand this? I thought Leica screw mount adapters said M39.


London, England
M42 is the 42mm screw mount for early SLRs, often called Pentax screw when in truth it should be called Praktica screw. Because the film plane on these cameras is different to the Leica the adapters are much deeper than the M39. So in a nutshell the Leica RF screw mount is 39mm usually called LTM but sometimes M39. Some old SLRs such as the Russian Zenit also use a 39mm thread but the lenses are different to LTM.

Here is a LTM adapter

View attachment 51983

Here is an M42 adapter

View attachment 51984

On my m4/3 camera I have both adapters and have successfully used Leica & Soviet M39 (LTM) lenses and Pentacon M42 lens with great results