Cross-Platform... Adapting to old lenses...

In the digital age, one cannot resist the ability to do more with their camera. I know I can't.

Having said that, there are many individuals out there that have taken to adapting or using adapters for their own lenses. In some rare cases, the camera's have been adapted to take older lenses. The advantage of adapting is that you are able to re-purpose your old film lenses. The disadvantage is, not all lenses are adaptable - but there's two sides to that unfortunate coin.

One is that, the register distance is quite different. The other is, the image circle is different.

Now, there are some cases, where a smaller lens can be adapted to a larger mount, but the results aren't always inspiring. This is the rarer of the two cases unfortunately. The more common problem in cross-platforming is the register issue.

Now, many mechanical adapters are out there, ready to allow you to mount Lens A to Camera B. For the most part, the more successful cross-platform matings belong to Canon, Nikon, Leica, Olympus and Pentax. I don't really know much about the other brands, but from what I understand, Nikon's and Olympus's are generally the other favourites. Leica cameras are often hideously expensive, so anyone wanting to adapt something to that kind of camera, most likely has the money to do so. :P

As for Canon... Canon has a register distance of 44mm, which is pretty short by modern standards, however, it is still longer than older standards. This means that, if you want to mount an older lens on a Canon, you're going to lose infinity focus. Alternatively, if you use a lens that's native to a longer register, you end up winning in infinity focus, but probably lose in minimum focus. No one really talks about that, so I'm only speculating on that. (stands to reason, right?)

Additionally to the shorter register, is the fact/possibility that older lenses will extend into the body, thus introducing the possibility of damaging your mirror.
There are examples of adapters for those kinds of lenses that maintain infinity focus and prevent mirror damage, but there is a cost... Image Quality. These kinds of adapters introduce (diopter) glass that acts as a weak teleconverter. In quite a few cases, I've read that 2/3rds of a stop are lost - sometimes more. No matter how good that lens is, losing that much is generally considered not worth the investment. Plus, the quality of said glass may be poor, so it ends up being money flushed down the toilet.

Bob Atkins created a nice little table with the different types of lenses out there that can and cannot be adapted to EOS mount. However, I felt that it fell short given the wide range of available lenses out there. (please note: i didn't say a wide range of awesome lenses)

CANON EOS	44.0mm	CAMERA WE'RE MOUNTING THESE LENSES TO
BODY	REGISTER	NOTE		BODY	REGISTER	NOTE
CONTAREX	46.0mm	ADAPTABLE		CANON SCREW (M39)	28.8mm	NAWM
LEICA R	47.0mm	ADAPTABLE		CANON FD	42.0mm	NAWM
NIKON	46.5mm	ADAPTABLE		FUJICA X	43.5mm	NAWM
OLYMPUS OM	45.5mm	ADAPTABLE		KONICA F	40.5mm	NAWM
PENTAX K	45.5mm	ADAPTABLE		MINOLTA MD	43.5mm	NAWM
PENTAX M42	45.5mm	ADAPTABLE		MINOLTA SR	43.5mm	NAWM
YASHICA YUS/DSB	45.5mm	ADAPTABLE		MIRANDA	41.5mm	NAWM
CONTAX/YASHICA	45.5mm	ADAPTABLE		KONICA HEXAR	28.0mm	UNKNOWN
CONTAX RTS	45.5mm	ADAPTABLE		LEICA SCREW (M39)	28.8mm	NAWM
PRAKTICA / HANIMEX	44.4mm	ADAPTABLE		OLYMPUS PEN F	28.95mm	NAWM
K-mount	45.46mm	ADAPTABLE		HASSELBLAD XPAN	34.3mm	UNKNOWN
M42 Screw	45.46mm	ADAPTABLE		CONTAX RF	34.85mm	UNKNOWN
Olympus OM	46.0mm	ADAPTABLE		NIKON RF	34.85mm	UNKNOWN
Arriflex	52.0mm	ADAPTABLE		CANON R/FL/FD	42.0mm	UNKNOWN
Mamiya 645	63.3mm	ADAPTABLE		PAXETTE	44.0mm	UNKNOWN
Pentax 645	70.8mm	ADAPTABLE		SONY ALPHA	44.6mm	PA
Kiev 60/Kiev Six	74.1mm	ADAPTABLE		Nikon F	46.5mm	ADAPTABLE
Hasselblad/Kiev88	82.1mm	ADAPTABLE		Contax N	48.0mm	UNKNOWN
T2 MOUNT	55.0mm	ADAPTABLE		Mamiya RZ	105mm	UNKNOWN
RICOH BAYONET	45.5mm	ADAPTABLE		Mamiya RB	112mm	UNKNOWN
PETRI BAYONET	45.5mm	ADAPTABLE
Legend:	NAWM	NEEDS ADAPTER WITH MULTIPLIER
	PA	POSSIBLY ADAPTABLE

Please note: the availability of adapters isn't exactly solid across the range. Some may need hunting.

If you're a lucky soul that has an older lens that has amazing optics, but is one of those listed as not having the ability to be adapted, don't despair. There are talented individuals out there that can modify the mount of your lens so you can continue using the lens on your dSLR. The more commonly modified/updated lenses tend to be Canon FD's, because there were many amazing lenses made back in the day.

Now... the Cavaet!

Some lenses, while they can be used with an adapter, they cannot be used on Full Frame cameras. They can only be used on crop cameras (EF-S mount [rebel series and xxD]). Always check to make sure that the lens in question is compatible in this way.