Use M42 Lenses with a Canon EOS DSLR

As an alternative to expensive lenses, many people have fitted M42 (commonly called "Pentax screw thread") lenses to their Canon DSLRs. M42 lenses are plentiful, and often very inexpensive in comparison to their more modern counterparts, as they were fit to a ton of '60s and '70s 35mm SLRs. Unlike other mountings, they have a flange focal depth very close to that of the EOS, which means you don't lose the ability to focus to infinity.

These aren't usually useful for sports photography that requires rapid focusing, for example, because they are almost without exception manually focused.[1] Nor are they particularly good for quick snapshots, since shots can take a little while to set up. But sometimes, economy is important, or you might happen have a bunch of M42 lenses sitting around. Maybe you just want to see what photographs old lenses will take. If so, then it may be worth your time to try using one with your EOS digital SLR.

Steps

  1. Screw the adapter onto the M42 lens. This is straightforward enough; but be gentle for the first turn or two, so as not to cross-thread either the lens or the adapter.
  2. Align the red mark on the adapter with the red mark on the camera body. The lens (or rather, the adapter attached to it) should drop into place with no real effort, just like any Canon lens.
  3. Turn the lens and the adapter clockwise until you hear it "click" into place. Again, this is the same as fitting any lens to the Canon.
  4. Set the mode dial on your camera to Av (aperture priority) mode. Since the camera will have no way of controlling the aperture of the lens, this will be the only one that works (except manual (M) mode, which may be more complex than you want to use). "Aperture priority" means that exposure control will be done by the camera adjusting its shutter speed according to the aperture you have chosen.
  5. Set up dioptric correction. Given that you will be focusing manually, [1] it is essential that the view through the viewfinder is as sharp as possible, and you may not have bothered with this if you had autofocus to fall back on. Set your lens to focus at something a known distance away (or, simpler, focus your lens to infinity and aim at something a little further away than the next closest distance on your lens). Look through the viewfinder, and turn the dioptric adjustment dial one way or the other until the picture is no longer fuzzy.
  6. Set your lens to "Manual" (M) with its manual/auto switch. With a normal M42 camera, in "Auto" mode, a lever inside the camera would depress a pin on the back of the lens to stop it down to your selected aperture during metering, or when taking a photograph. Of course, the EOS camera body has no such linkage, so you'll need to stop down manually.
  7. Set your lens to its widest aperture, i.e. smallest f/number. You need to do this to make the screen as bright as possible for focusing.
  8. Focus on a well-lit subject. Since you often won't have any aids on your mirror, like a microprism ring, to help you focus accurately, this can be awkward. It sometimes helps to keep turning the ring until you are in focus, turn it a little further till you're out of focus, then very slowly turn it back. Once you're in focus, stop your aperture down by a couple of stops; this will give you a greater depth of field to compensate for the inevitable focusing error, however small.
  9. Take photographs. Take several photographs of well-lit subjects. Look at them on your LCD screen; chances are that you will find your lens consistently under- or over-exposes in certain conditions (for example, the Pentacon 50mm 1.8 tends to cause the camera to overexpose by about +1 to +2 EV), so you will need to...
  10. Set up exposure compensation. Exposure compensation on an EOS maintains automatic control over the shutter speed, but will underexpose or overexpose the picture by a given amount. Experiment with varying degrees of compensation and take as many photos as you need to get it right.
  11. Get out there and start taking more photos. Every lens has limitations, and many of them will have unique strengths. Ultimately, you can only find out what these are by taking as many photos as you can.

Things You'll Need

  • A Canon EOS digital SLR. These instructions may or may not be entirely useful with EOS film SLRs.
  • An M42 adapter ring. There are varying opinions on whether the much more expensive rings with the AF-confirm chips are really worthwhile.[2]
  • An M42 lens. You can find reviews and specifications of manual focus lenses, including a huge number of M42 lenses, at the Manual Focus Forum. When picking any lens, it is very important that you pick one with a manual/auto switch. Otherwise, depending on your adapter, you may be stuck with the diaphragm wide open all the time.

Related Articles

Sources and Citations

  1. ↑ 1.0 1.1 There are some very rare exceptions, such as the Sigma 55-200 autofocus zoom, which had all the autofocus equipment built into the lens itself. But such exceptions are very few.
  2. ↑ For example, see this horror story about a bad AF-confirm chip killing a 20D body.
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