As an alternative to expensive lenses, many have mounted an M42 lens (commonly referred to as a “Pentax thread”) on their Canon DSLR. The M42 lens is widely available, and often far less expensive than the more modern equivalents made for many 35mm SLRs from the 1960s and 1970s. Unlike other mounts, it has flange depth of field very similar to the EOS one, which means that it retains the ability to focus indefinitely.
It is not a useful lens for sports photography that requires fast focusing, for example, because it almost always has to be done manually. Nor is it particularly suitable for instant shots, as the shots take some time to set up. But sometimes, the savings matter, or you might have a bunch of M42 lenses around. Maybe you just want to see what photos the old lenses take. If so, then it might be worth trying one with your EOS digital SLR.
Steps
Step 1. Screw the adapter onto the M42
This is enough; but be gentle in the first few laps, so as not to damage either the lens or the adapter.
Step 2. Align the red mark on the adapter, if there is one, with the one on the camera body
The lens (or rather the attached adapter) should snap into place effortlessly, like any Canon lens.
Step 3. Turn the adapter and lens clockwise until you hear a “click”
Again, it's the same process as the other lenses.
Step 4. Set the mode to “AV (Aperture Priority)”
Since the machine will have no way of controlling the lens aperture, this will be the only mode to work (except the manual (M), which however may be too complicated). "Aperture priority" means that the exposure will be controlled by the machine by adapting the shutter speed based on the chosen aperture.
Step 5. Set Diopter Correction
Since you will be using manual focus, it is essential that the view from the viewfinder is as sharp as possible, and you may not have had to do this with autofocus. Focus the lens on something at a known distance (or more simply, focus at infinity and point to something a little further away than the thing closest to the lens). Look in the viewfinder and change the diopter setting from one until the image becomes clear.
Step 6. Set the lens to "Manual" (M) with the manual / automatic lever
With a normal M42 camera, in "Auto" mode, a lever in the camera would release a dot in the back of the lens to lock it onto your selected aperture when focusing, or taking a photo. Of course, the EOS camera body does not have this connection, so you will have to lock it manually.
Step 7. Set the lens to the widest aperture, or the lowest "f /"
This is to make the screen as bright as possible for focus.
Step 8. Focus on a well-lit subject
Since you will often not have aids on the mirror, such as a micro-prism ring, to focus more accurately, it could be a bizarre experience. Sometimes it is useful to keep turning the ring until you are in focus, turn it "a little more" until it comes out of focus, and then bring it back. Once in focus, lower the aperture a couple of stops; this will give you greater depth of field to compensate for the inevitable focus error, however minimal.
Step 9. Take photographs
Take lots of photos of well-lit subjects. See them on your LCD screen; your lens is likely to under- or over-expose continuously under certain conditions (for example, the Pentacon 50mm 1.8 tends to over-expose the camera by about + 1 / + 2 EV), so you'll have to …
Step 10. Set an exposure compensation
Compensation on an EOS maintains automatic shutter control, "but" under- or over-exposing the photo by a certain amount. Experiment with varying degrees of compensation and take as many photos as you need to learn.
Step 11. Exit and start taking more photos
Each goal has limitations, and many have unique strengths. Ultimately, you can only find out by trying them on and taking as many photos as you can.
