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05-07-2011, 12:25 PM   #1
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Join Date: Aug 2007
Location: Shropshire, UK
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M42 Adaptors. The good and bad.

Pentax to M42 adaptors, always a subject for some discussion, with the general concensus recommending the genuine Pentax adaptor, and a few of us also recommending good quality adaptors from independant manufacturers. I've been a champion of the British made adaptors by SRB Griturn that I've always found to be reliable and excellent quality, and cheaper than the genuine ones.
But I think we all agree the cheap and nasty ones should be avoided

I have a genuine Pentax adaptor, a few SRB adaptors, and the other day I found a cheap unknow brand adaptor at the local camera fair. When I looked at it closely I could see that there was some brass showing through the plating, so I got a good adaptor from my bag and compared them. I didn't like what I saw, but I paid for it because I wanted to see what was happening to make the wear on the adaptor.

Once I got it home I tried it with an old lens on a camera mount that I have removed from an old ME Super that I use for experimenting. When I fitted the lens and adaptor to the camera mount the adaptor span one notch and came out, there was no dead stop for the adaptor.
And this is why.

The unknown adaptor is machined differently in this one important place, there is a curve instead of a right angle at this point, and the curve allows the spring steel stop in the camera mount to ride up allowing the mount and lens to over rotate and come off.


The SRB and the Pentax adaptors have a right anle that contacts the stop firmly and will not allow it to ride up.





Here you can see the three adaptors mounted on the camera mount from behind, and the first picture shows the stop at the point where it is going to ride over the outer edge of the adaptor.


Both the SRB and the Pentax adaptors are made so the stop in the mount works in the same manner as it does on Pentax lenses, all of mine have the right angle and not a curve.




My unkown one is going in the bin, every time I tried it on the lens mount, and it rode up over the stop, a tiny sliver of brass came off and could be seen with a loupe on the camera mount stop, and that's something I don't want in my camera.

05-07-2011, 11:37 PM   #2
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Join Date: Jan 2010
Location: Southern England
Posts: 495
Very nice illustration of one of the 3 key design features for a decent adaptor! The other 2 are:

Make the design a loose fit in the camera.

Allow the tightening of the lens to get rid of the looseness by making the adaptor slightly recessed relative to the camera's mounting plate.

It sounds as though the SRB adaptors may have been designed with these features in mind. Well done SRB, and well done you for finding a decent alternative to Pentax!
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