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

There is a FotodioX adapter which has aperture control and maintains infinity focus through an optical element.

FotodioX Pro Lens Mount Adapter for Sony A Lens to Canon EF Mount

Answer 3

It's the mount throat diameter difference that causes the problem. The throat diameter for A-mount is 49.7 mm. Canon's EOS mount is 54mm.

The diameter of the Minolta AF/Sony A-mount is smaller than that of Canon EF. Which means the lens mount itself can be too wide to actually fit inside the mount and has to sit completely on top of the adapter. The farther out the lens sits, the more far focus range you lose with a simple ring adapter that has no glass elements.

The depth difference (0.5mm) is also too thin for someone to reliably machine a ring to a consistent thickness for adapting the bayonet differences.

So, even though the registration difference (the distance the lens is held from the image plane) would theoretically allow A-mount to adapt the same six mounts (Nikon F, Leica-R, Contax/Yashica, Olympus OM, M42, and Pentax K) Canon can, in reality you can only adapt M42 and Leica R with simple ring adapters. M42 is the only one with a small enough throat diameter to fit appropriately, and Leica R's registration distance is so deep, the diameter doesn't matter.

In addition, Canon EOS-mount lenses don't have aperture rings. So adapting one to A-mount would probably mean loss of aperture control unless the adapter can translate the electronic communication from the camera body as well.

Answer 4

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.