The M42 "universal screw thread" lens mount was used on film cameras from Pentax, Praktica, and others for decades...  so a lot of great old lenses are available at modest prices. A lot of people are buying adapters (usually $5 to $30) that allow M42 lenses to be used on modern DSLRs made by Canon, Olympus, Pentax, Sony, etc.

Physically mounting an M42 lens on your DSLR is easy with an appropriate adapter, and of course the lens will have to be focused manually, but there is also a little matter of how one controls the lens aperture. The aperture is the thing shown in the photo; a diaphragm consisting of a set of blades that allow the amount of light passing through the lens to be reduced to the desired level, which also allows a controlled increase in the depth of field.

There are three basic types of M42 aperture controls , listed here in chronological order. The catch is that all three types can have no response to turning the obviously marked aperture ring -- without being broken. This little instructable is about how to make aperture control (f/ number setting) on each of these types of lenses work on your DSLR.

Step 1: Preset Aperture Control

Before the middle 1960s, M42 lenses typically had preset aperture controls, as does the Soligor 135mm f/2.8 shown here. Preset lenses typically have two separate aperture control rings near the front of the lens.

One of the rings is used to set the smallest aperture (minimum opening and largest f/ number) that you might like to use for taking a photo. This ring is usually marked with f/ numbers and often will have detents that allow it to hold its setting. Turning it usually doesn't get any response from the aperture blades.

The second ring may also be marked with f/ numbers, or sometimes simply with the letters "o" and "c," or it may be entirely unmarked (as in the photo). It usually doesn't have any detents, but turns smoothly. Turning this second ring toward "c" (close) allows the lens aperture to actually be closed to the f/ value set on the first ring, whereas turning toward "o" opens the aperture.

A few preset lenses (e.g., Volna-9) actually use a single ring at the front of the lens for both functions. Pressing that ring toward the camera body allows setting the stop.

Because preset lenses were completely manual, they still work precisely as intended on modern DSLRs. Actually, they are the easiest to use on a DSLR. You focus with the lens wide open and then twist the second ring to stop-down to the desired aperture just before taking the photo.

These lenses often will have the aperture blades far out on the lens body, away from the camera body. More significantly, there are often many aperture blades, giving a very circular openning. Unlike other lenses, a little oil on the blades generally is not a problem for a preset lens.  Friction is pretty high, but your hand turning the second ring can provide plenty of force.

<p>Very interesting piece. It certainly got me thinking. However I respectfully think this is setting about the task in a manner which is fundamentally flawed. Why buy an adpator if you are still going to hack the lens anyway. It really is like buying a proverbial dog and barking yourself. Surely a better approach would be to hack the adaptor instead?</p><p>Firstly adaptors are simple and cheap. If you break it, then it is easily replaced. Whereas lenses are more expensive and becoming increasingly rarer with the relentless march of time. And modding the adaptor is actually really simple:-</p><p>Most adaptors come apart. The 42mm screw part is retained by three grub screws. loosen these grub screws (no need to remove them completely) and remove the threaded part. Measure the adaptor's inside diameter - i.e the diameter of the bit that the threaded part fits into. In my case using the Fotga adaptor, this was 51.5mm. </p><p>Find a piece of thin flexible plastic. between 0.5mm and 1.0 mm thick. Bit of left over blister pack would probably do. Cut a circle 51 mm in diameter. Measure the centre of the back of your lens. Probably a bit under 34mm. So cut hole on plastic disk with trimming knife 34mm in diameter. This effectively makes a washer 51mm OD by 34mm ID. Clean off any swarf. Place washer in adaptor. Refit threaded part on top of home-made washer. Tighten grub-screws. Et voila! Cheap, clean, reversible and no damage to the lens!</p>
<p>This is simply a way to make the ledged adapter (option 1 in Step 3), which you can easily just buy at no higher cost than the un-ledged version -- they're no longer hard to find (they were when I wrote this Instructable). I guess yours is nicer in that the plastic is flexible, which a metal edge surely isn't.</p><p>The correct adapter hack would be to have a moving piece that depresses the pin on demand. That's actually not hard to do now that I've been designing and 3D printing my own adapters for a couple of years. Maybe it is time for me to update this Instructable...?</p>
Thanks for the micro surgery tip. I have been using my helios 44m-2 always wide open, and couls only think of ways to hold the pin from the outside. Now what I did was to remove just the four screws the hold the first plate of the lens, gaining acces the the pin from inside. I rolled up a 2mm broad piece of tape on it and voila, it worked perfect...
Thanks a bunch for this tutorial. I recently purchased a MIR 1B m42 lens for my DSLR and was a bit confused after finding out it had a preset type aperture control. This tutorial explain very clearly how to use it. Thanks again!
to mod an automatic lens(without M switch)simply open the back of the lens and stick 3mm of rubber(i used a piece of silicone tube);<br /> i did it with all three of my Helios 44m and works very well!<br />
The minor surgery you suggest is known to work for auto Helios 44 lenses and at least <em>some</em> other M42 lenses:<br /> <br /> <a href="http://forum.manualfocus.org/viewtopic.php?id=913" rel="nofollow">/forum.manualfocus.org/viewtopic.php?id=913</a><a href="http://www.pentaxforums.com/forums/pentax-slr-lens-discussion/51749-m42-auto-only-lens-modification.html" rel="nofollow"><br /> www.pentaxforums.com/forums/pentax-slr-lens-discussion/51749-m42-auto-only-lens-modification.html</a><br /> <br /> Unfortunately, some lenses have small parts that can fall out of place if the wrong screws are removed....<br /> <br /> Incidentally, opening the back of the lens as you discuss and pushing the pin is one of the surest ways to break the glue bond to undo the method I discussed.<br />
it's true but if you use caution and precision all the m42 lens are easy to open;i have a lot of them and i've opened most of them in all their parts for repair without problems.<br />

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Bio: I'm an Electrical and Computer Engineering Professor at the University of Kentucky. I'm probably best known for things I've done involving Linux ... More »
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