One of the problems of using manual focus lenses on Canon EOS bodies is that focus confirmation is disabled. When AF lenses are used - even in manual focus mode - all Canon EOS bodies confirm focus with a green dot in the viewfinder and an (optional) audible beep.
However when non-EF compatible lenses are used via an adapter, the focus conformation is disabled. Canon didn't have to do this, but they chose to. Unless the camera thinks that an EF compatible lens is attached to it, it switches off both visual and audible focus indication.
If the camera could somehow be tricked into thinking an EF lens was attached, then the focus confirmation indicators should work with any lens attached. Well, this is just what the developer of this special M42 adapter has done.
The adapter has the same contacts on it as a regular EF series lens, and presumably also contains a ROM chip which supplies information to the camera body that tells it a lens is attached. In fact from the EXIF data recorded when an image is taken using this adapter, it tells the camera that a 55mm f1.4 lens is attached.
The adapter is shown in the image below. It's finished in a dull black. The material feels too heavy to be aluminum and the black finish doesn't seem to be anodization. I think the adapter is made of brass and the black finish is produced by a chemical process. It can be scratched off and probably will be in places when the adapter is used. Overall the adapter seems well made, but the finish is "Russian Industrial". Don't expect a work of art!
Below is an image of the most important part, the plastic(?) housing which holds the electronics and the contacts which transmit information to the camera body.
So how do you use it? Simple. You screw the adapter onto any lens with am M42x1 (Pentax Screw) mount, then you mount the adapter on the camera. The adapter depresses the aperture pin found on some M42 lenses and so the lens operates in stop down mode. The pin is used with some cameras to stop the lens down when the shutter is released, but to allow focusing at full aperture. However when the pin is always depressed (as with this aperture), the lens is stopped down to the indicated aperture all the time.
When the lens and adapter are mounted on the EOS body, the body will readout an aperture of f1.4. This is the value it's reading from the ROM chip in the adapter and has nothing to do with the lens attached to the adapter or the aperture to which it is set. The readout will always be f1.4, whether lens is attached and whatever aperture that lens is manually set to.
In Av (aperture priority) mode the camera will select the correct shutter speed for whatever aperture the lens is set to. The EXIF data in the image will always record 55mm and f1.4. Just ignore the f1.4 in the display. In manual mode you set the shutter speed yourself and use the in-camera metering to know when you've got exposure right. You can't use Tv mode (shutter priority) because the camera cannot control the lens aperture.
When manual focusing any lens there's always a small region over which the focus confirmation light stays on. Canon define focus as "within the depth of field" for cameras with low precision sensors and "within 1/3 of the depth of field" for cameras with high precision sensors. Since the ROM tells the camera the lens is f1.4, the high precision AF mode (normally activated by the use of lenses faster than f2.8) should be enabled in cameras which support it. Of course it would also be activated when lenses slower than f2.8 are used with this adapter, but in practice this didn't seem to cause any problems. Most of the M42 lenses likely to be used are wideangle to midrange primes anyway, most of which will be f2.8 or faster.
I tested the adapter with several M42 lenses using an EOS 20D, a Peleng 8/3.5 circular fisheye, a Zenitar 16/2.8 full frame fisheye, a Pentax 50/1.4, a Pentacon 135/2.8, a Tamron 500/8 mirror lens and a 500mm f5 refracting telescope. With the 16/2.8, 50/1.4 and 135/2.8 the camera gave accurate focus indication from wide open to f5.6. At f8 focus confirmation still worked about 90% of the time but sometimes didn't if the subject was low contrast. At f11 there was no focus conformation signal. This is consistent with Canon's limit of f5.6 for AF lenses on the EOS 20D. It's possible that focus confirmation might be better at f8 using an EOS-3 or other body that gives AF to f8 (e.g. the EOS 1Ds Mk II). Again I'll remind readers that the focus confirmation is all done in the EOS body using the Canon electronics. None of the work is done by the adapter. All the adapter does is turn the focusing system on.
Focus confirmation with the 500/8 mirror lens was very hit and miss. With a bright, high contrast target you could occasionally get focus confirmation, and when you could get it, it was accurate as shown below:
However most of the time the focus confirmation failed and there was no electronic indication of focus. At least on a 20D I'd say the the use of this adapter with a 500/8 mirror lens won't give you much focusing help.
Focus confirmation using the 8mm f3.5 fisheye wasn't very good either, but this isn't unexpected. Focusing such a lens is difficult because the depth of field is so large that things look in focus all the time. It may also be that the focusing optics in the camera aren't designed to cope with the strange optical system of a circular fisheye lens. Normally you'd just scale focus it and that would be good enough. Sometimes it would indicate focus (most often on closeup rather than distant objects) and when it did it seemed to be accurate, but like with the 500/8 don't expect the focus confirmation to give you much help with focusing an 8mm circular fisheye lens.
The adapter is available from RuGifts.com for $89. Shipping to the US is $10 and takes a couple of weeks. My adapter came wrapped up in a brown paper parcel, tied up with string, but it got here safely. There is a small leaflet enclosed (the "instruction manual"), but it's written in Russian. Luckily it doesn't tell you anything you don't already know after reading this article!
You can search Ebay for adapters, both with and without focus confirmation. Use the search term "EOS Adapter AF Confirm" to select only those adapters which allow AF confirmation
Regular (non focus indicating) M42 and Nikon to EOS adapters are available though Amazon: