Several sellers on eBay sell mount adapters wich purport to allow the mounting of M42 screw mount lenses on various camera bodies. This is great news if you want to use less expensive manual lenses on your new digital SLR, but even better it is not so well known that many of these older lenses from various manufacturers are incredible performers, easily matching and often exceeding the lens supplied with your camera!
Not all adapters work well with some cameras. Apart from quality issues, which are important in order to avoid possible damage to your nice new camera, some are just plain wrong!
Registration distance. The problem.
The problem is that many different cameras have different registration distances. This is simply the distance from the front of the film or sensor plane to the cameras lens mount surface. For M42 this is 45.5mm exactly.
If the lens you intend to use is desinged to for a camera mounting system with a different registration distance then the lens will not focus properly at infinity. If the registration distance is too small the lens will focus past the infinity point. That's not too important, as it will include infinity focus withing the it's range, although the distance of closest focus will be further away.
Much more of a problem is if the lens is positioned too far away. This will mean that the lens cannot be focused at infiniy at all.
Cameras which will have these problem when using M42 lenses include Nikon (46.5mm) and Canon (EF mount ie EOS cameras with 44mm)
This can be solved for shorter registration distances, by adding the distance to the thickness of the adapter, greater distances can be accomodated by using an adapter which contains an extra lens element which corrects the focal distance of the lens. This also affects the actual focal length, but also it must be remebered that the extra lens has to be a high performance piece of glass if it is not going to adversely affect the performance of the lens.
Unfortunatley a lot of the adapters sold on eBay do not account for registration distance mismatches well. Worse, some even introduce a registration distance error where there was never a problem!
For example the registration distance used by the Pentax K family of mounts is exactly the same as the older M42 mount registration distance. It was desinged that way, along with an adapter for M42 lenses quite deliberately, because older Pentax lenses used the M42 mount. This adapter is designed to sit inside the K mount, so thet the front surfaces are perfectly flush with each other. I have seen many M42 to Pentax K mounts which sit on top of the K mount, actually introducing a focusing error! (Often at a higher price than the original part, which last time I looked, is still available from Pentax.)
So how do you know which adapter is best for you?
First find the lens regitration for your camera or mount. There are plenty of websites which list these. Just search for 'Lens registration distance' etc. Once you know this you will be able to tell whether the adapter needs to compensate for the difference or not, and if so my how much. Asking the seller what the registration distance for the adapter is will soon help pick out the numskull seller who really does not know what the hey he is talking about. (and therefore probably about to sell you a pup!)
Examination of the picture (if any) in some cases will imediately tell you the registration distance will be worng. In the Pentax example, above the presecence of a flange, or a large diameter is a dead give-away.
What if you can't find a suitable adapter on eBay?
Well in the first place, you probably can, so don't stop looking. They are a little less common than the lenses themselves of course, but that's because you generally only need one adapter to fit a whole family of lenses. (Especially M42!)
Ocassionally though, you simply can't wait, or maybe it's a bit unusual. However do not despair. There are companies who specialise in this sort of thing, and at least one company I know of one that will even make one to suit your personal requirements for very little money!
Do your research before buying.
In all cases a bit of research on the internet using search terms like 'M42 Registration' or 'lens adapter register' should always be done before buying from eBay, as some adapters are better than others, and if they introduce a registration error, may even be useless even if they do join the lens to the camera.