Originally Posted by MacDaddy
I find it interesting in reading this discussion that no one has pointed out that several manufacturers, including Canon and Nikon, but most noteably Pentax, have trumpeted the fact their older FILM designed lenses apparently work quite well with their DSLRs. I had a Pentax ist DS prior to switching back to film and was able to successfully use lenses as old as 35 years old with the proper adapters! Some of their prime Limited series lenses are outstanding, and this on an APS-C size sensor. The only "curse" was the blasted crop factor (1.5x), but it was liveable with for me.
Regarding backwards compatibility, CanNik don't even come close to Pentax which offers manual metering and Av mode even with M42 lenses via a cheap adapter.
EF lenses are compatible with Canon AF bodies. That's it. Not FD mount lenses and certainly not LTM lenses.
Nikon is worse still. No metering is offered with manual focus, non-CPU lenses except with the most expensive bodies and the FM3A. Even within the AF system, not every lens is compatible with every body (and vice versa). The F mount unchanged for 50 years? My ass!