Why I can't find an A-mount to Canon EF Adapter?

Question

I looked online for an adapter from A-mount to Canon EF but I couldn't find. The Minolta/Sony A-mount flange focal distance is 44.50mm and Canon EF flang focal distance is 44.00mm, in theory A-mount lenses can be used with a proper adapter on Canon EF system, but I can't find one, I'm curious why?

Answer 1

First of all, this leaves only .5 mm for the adapter, which isn't a lot. With a mount that's a lot smaller in diameter most of the adapter could sit inside the EF mount ring, and you could probably do it. From what I recall of the diameters, they're similar enough that this would be extremely difficult, if possible at all (and I'm leaning toward "probably not possible").

Second, EF mount uses an electronic connection for both aperture and focus, where A mount uses a mechanical linkage for aperture, and either mechanical or electrical for focus (but electrical was introduced relatively recently, so most older lenses being adapted would probably be mechanical).

Since (most) A mount lenses don't have aperture rings, you'd have to build an aperture control into the adapter. If it was purely mechanical, you could do stop-down metering. In theory, an adapter with some sort of built-in servo could receive the electrical signals from the body and translate them to mechanical movement for the lens. Given the small difference in flange distances, neither of those would probably be simple or straightforward -- but without it, you'd have no aperture control, so you could only shoot at the lens' minimum aperture (typically f/22) -- pretty useless. You could build a fixed stop into the adapter to always hold the lens at, say, f/8 I suppose, but it would still be quite limited, even at best.

Answer 2

An adaptor will always take up some space, so whilst the A-mount flange-focal distance is 44.5mm by the time you have a 2mm adaptor in the way your lens will be mounted 46.5mm from the sensor, which is probably enough to lose infinity focus with some lenses.