Digital cameras, like all other "tech" fields, have advanced extremely fast. From essentially not existing 20 years ago to the rapidly advancing market we see today. I shall try not to terribly upset serious photographers by saying lens technology is not advancing, but it is certainly not advancing at quite the same rate. A digital camera from 15 years ago is almost entirely useless today, glass from 35 years ago is however a different story. The biggest metamorphosis that SLRs have gone through is arguably* not the change to digital but rather the change to auto-focus more than a decade earlier. That is when some of the biggest changes where made, when new lens-mounts appeared and old ones disappeared. This leaves us with many good lenses that suffer from being both manual focus and designed for an obsolete mount. i.e. useless. A waste of good glass.
Or are they?
If you are prepared to with manual focus, then no, this is not the case at all. These lenses have no motors, and no way of electronically communicating with the camera (and nothing to say, had they the means to say it) so obviously they are never going to auto-focus, however physically fitting them to a current lens mount is no more complicated than using the right adapter. Oh the joy of fitting together two pieces of metal - use a third piece. Like this one:
Pictured: A piece of metal.
A Fotodiox M42 to EOS adapter. M42 (Pentax screw) being a very popular old manual focus 35mm film SLR (and rangefinder) screw type mount and EOS - more correctly EF or EF-S - being the current electronic bayonet mount used by Canon EOS auto-focus film and digital cameras.
Sadly its not entirely that simple (when is it?), complications arise from the distance from lens to the sensor, if you can't get this right the lens wont focus, or at least not to infinity. I won't try to explain this here but the end result is that some moderns are utterly unsuitable for using adapters (like the Nikon F-mount) and others will require longer/deeper adapters. (Like micro 4/3 but more on that in another post.) The Canon EF mount however lends itself very well to this endeavor, so I bought myself a cheap Canon EOS 300D.
Historically significant as the first sub $1000 DSLR, today just a cheap camera with a nasty plastic case. Notice the EF-S bayonet mount on the camera, next to it is a matching Canon EF-S 18-55mm kit lens. At the of the lens you will see the electronic ("CPU") contacts, you will see the corresponding contacts on the camera at the bottom of the mount. The EF-S is an electronic mount - all communication between the camera and lens occurs through those contacts. (Unlike, say an Nikon F-mount.)
Here the adapter is locked into place. Actually, because EF-S is bayonet and the M42 lens is a screw mount, its easier to leave the adapter attached to the lens.
And there is our vintage lens, in this case a Chinon 55mm f1.7 M42, the same lens fitted to the EOS 300D in the first picture. As you see there are no CPU or mechanical contacts on the adapter. The only control that the M42 lens expected to receive in any case was when to stop-down the aperture - the Canon EF does this electronically so the function is lost. Aperture control occurs via a beautifully machined ring near the rear of the lens, focusing at very small apertures can be hard due to the viewfinder (already poor on the 300D, like most autofocus SLRs) getting rather dark, this one just has to live with. Likewise the poor viewfinder can make focusing itself hard, especially with the lens at f1.7. Practice is needed here, an alternate solution is to fit your camera with an old-school split image focusing screen. (Starting from around $25 from China - I am not sure yet how good these are.) The better viewfinders that higher end DSLRs possess could well make this task either - I guess I should have conducted my experiments with a EOS 20D, not a 300D...
On the Canon you get through the lens metering, meaning you can use either manual or aperture-priority modes (you control the aperture yourself from the lens) and you will probably get the correct exposure. I say probably because, especially with a fast lens like this Chinon, you can easily "run out" of workable shutter speeds in bright light.
So obviously there are drawbacks. The rewards? Well aside from the extreme cheapness of these lenses - that Chinon 55mm 1.7 cost me 120kr (less than $20) and I have a Wiestar 35mm and 135mm that cost me even less - there is some sense of satisfaction in using these old lenses, in addition to the fact that the lenses themselves are often beautiful. The Chinon is a work of art compared to the frankly icky plastic Canon kit lens pictured above.
To summarize
Using M42 lenses on Canon EOS:
Aperture preview: No
Camera aware of aperture: No
Metering: Yes
Available modes: M, Av (Manual and Aperture Priority if you don't speak Canon.)
*I argue that it is.
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