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Disabling 'automatic aperture' on M42 lens
Posted By: Frogonastring, 06-19-2014, 03:49 AM

Hi all,

I've got a lovely Cimko MT 28mm f2.8 M42 lens that is really good and has sharp optics - but rather irritatingly doesn't have an auto/manual switch to allow it to be used on more modern cameras that don't have the lever to depress the aperture pin.

This makes using it on my DSLR a nightmare - f2.8 is lovely to have, but the DOF is too shallow for landscape work, and there's no way of changing the aperture.

I've just had a little fiddle with it and thought I'd share with you my instructions for non-permanently and completely reversibly adjusting old automatic M42 lenses to allow manual aperture control.
This obviously depends on the lens design, but here the fix was quite simple.

This is probably quite a common task - but I thought it might be useful to people who had the same issue/style lenses and wanted to use them to their full capacity on more modern cameras.

There's a Google+ gallery with this information too, here: link!
I've also tried to attach the photos below...

Step 1: The lens as it stands - I've removed the three screws that hold the metal cover around the rear lens element.

Step 2: Remove the metal cover - mine was held fairly well by friction, but using a blob of blue-tac to stick to it to lift it out worked well.

Step 3: Rotate the metal cover by roughly 33, so that the screw holes line up successfully but the indent for the aperture pin is now in the "wrong" place, and so the pin is covered.

Step 4: Screw the metal plate back down, pressing the pin down as you go. I found it worked best doing a few turns of each screw in a circle to keep the pressure even. There's a slight lift in the metal plate on the side where the pin is, but the screws are tight and won't work their way loose.

That's it! The aperture now works manually without having to have a lever to depress that pin.

My only question would be...are there any downsides to keeping the pin depressed like this? is it likely to stretch/damage the springs inside the lens?

Hope this is helpful!

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06-21-2014, 05:02 PM   #16
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QuoteOriginally posted by Dicknet Quote
Forgive me for asking, but why don't you save yourselves a whole lot of trouble and buy lenses without automatic diaphragm's? In fact, why do you use these old lenses at all? Your Cameras cannot communicate with them, The digital Cameras cannot set the apertures and you are left with an aperture priority exposure system. There is absolutely nothing wrong with that but why buy the latest technology and then take half a dozen steps backwards? I use a Pentax SV from the 1960's and a Pentax SP500 from the 1970's. I also have a DSLR and a Digital Bridge camera but to mix the two systems makes no sense at all. As a matter of interest, my oldest Camera dates from 1901.

Pentax SMC DA 50mm F1.8 - $270
Pentax A 50mm F1.7 - $70
Pentax A 50mm F2 - $50
M42 50mm F~2 - $20

06-22-2014, 09:20 AM   #17
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Maybe I should buy an adapter for my Canon DSLR and use some of my Pentax lenses on it. You never know, I may even enjoy it.
06-22-2014, 02:54 PM   #18

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QuoteOriginally posted by Dicknet Quote
Maybe I should buy an adapter for my Canon DSLR and use some of my Pentax lenses on it. You never know, I may even enjoy it.
Well, that's another reason to do's fun.
10-29-2014, 08:55 AM   #19
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Excellent idea! Takumar are the best!

10-30-2014, 01:30 AM   #20
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At last! a sensible way of solving this problem without actually destroying a lovely old lens. I have taken a lot of flack in the past on this subject because I use Pentax Cameras from the1960's and I hate to see these lenses being taken apart and then being made unusable on another Camera. But you have given this a lot of thought and have come up with a reversible solution. I would just add that if all three holes don't line up you can always get a Dremmel and re- drill the flange.
02-25-2016, 08:55 PM   #21
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M42 Auto Pin -Electrical Wire Insulation

I saw this solution on another site, so I thought I would try it. Please see picture.

Works well if you have this type of mount...
Attached Images
02-26-2016, 01:10 AM   #22
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Deleted cos I answered this before - a benefit of old age is that I can enjoy posts many times as if it was new.

Last edited by Arjay Bee; 02-26-2016 at 01:17 AM.

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