Originally posted by woldsweather Ah that's dollars I'm in UK. What would it be called so I could search for it here. Would like to get it by next Tues. ideally.
Search for "M42 to Canon EF adapter".
I use two different M42 to EF adapters which I bought on U.S. ebay. One has an inner flange to depress lens mount stop down pins, and the other doesn't. The flanged adapter is helpful for lenses which don't have an Auto/Manual switch where depressing the mount pin is the only way to stop down the aperture.
Super and S-M-C Takumars have an Auto/Manual switch so a pin pressing flanged adapter isn't needed to stop down the lens. And if your 300mm is S-M-C, be aware that EF adapters with a flange may interfere with the S-M-C's open aperture metering hardware.
My S-M-C lenses will work if attached to a flanged adapter, but when the adapter is screwed on snugly, the metering hardware on the lens mount rubs against the adapter flange making the aperture ring difficult to turn.
And my 5D has never had a problem with the mirror hitting the rear of M42 telephoto lenses. I've had trouble only with 50mm and shorter lenses.
These are my two M42 to EF adapters - with flange and without:
And this is how my adapters appear when mounted on a S-M-C Takumar lens: