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I've been using a Sony DSLR/SLT (a57) for a number of years now and have recently purchased a old (but pristine) Minolta X-300 film SLR to have a go at manual black and white photography. I have a number of Sony A mount lenses that I (perhaps foolishly) thought I could use with the old bird but don't seem to be able to find an adaptor that would let me do it this way round.
Does an adaptor exist to mount modern Sony A-mount lenses to an old Minolta MD/MC body?
You can share your A-mount lenses with an autofocus A-mount Minolta 35mm film body, but not an MC/MD-mount manual focus one--that's a completely different mount, different registration distance, different bayonet/physical linkage. No electronic communication. While you could, theoretically adapt with a ring, you'd have issues with vignetting if your A-mount lenses are for 1.5x crop, not full frame, and the not-so-small detail of not having an aperture ring to set aperture (remember, no electronic communication between the lens and the body in MD/MC). So, you have no way to set the aperture on the lens.
These reasons are probably why you can't find an adapter. Adapting MD/MC to A-mount can work, because those lenses have aperture rings, and you're typically adapting a full frame lens to a crop body, so there's no vignetting.
You basically need a autofocus Minolta film body that was made in 1985 or later. Unfortunately, your X-300 was made in 1984. In 1985, the Minolta A-mount was introduced with the Maxxum 7000, so any of those subsequent film bodies should work just fine with your A-mount lenses. You just might have vignetting if the lenses are for APS-C, not full-frame.
See also: Can I use lens brand X on interchangeable lens camera brand Y?
Minolta/Konica-Minolta/Sony A mount cameras and lenses have a registration distance of 44.5mm.
The Minolta MD and MC cameras use the Minolta SR mount. Those cameras and lenses have a registration distance of 43.5mm
In theory one could use a 1mm thick adapter to place an A mount lens onto an SR mount body. The problem is that 1mm just isn't thick enough to leave room for the connections that need to be made between the adapter and lens and between the camera body and adapter.
Parts of MC/MD lenses such as the aperture levers and the bayonet lugs themselves extend for more than 1 mm into the camera. These parts on MC/MD lenses don't line up properly to fit through the opening in the A mount flange. This means that an adapter to use A mount lenses on a Minolta SR mount camera would need to be much thicker than the 1 mm difference in flange focal distance. That, in turn, would necessitate the need for optical correction to enable infinity focus.
It might be theoretically possible to use the flange off an old MD mount lens to adapt an A mount lens to an MD mount camera body, but it would necessitate removing the flange from the A mount lens and replacing it with the MD flange plus a 1 mm spacer. I've read that the holes for the screws that attach the flange to the lens in MD flanges and in A-mount flanges actually align.
So question 1 is, does such an adaptor exist (old camera MC to modern lens A)?
Doesn't seem to. I searched around a bit and didn't find one, and I presume you did as well before posting your question. Even if it's possible to adapt alpha lenses to the MC mount, it probably doesn't make economic sense for someone to design, manufacture, and market such a thing. How many customers are there likely to be for an adapter that lets you use new lenses on an old body? The interest is generally in the opposite direction: people wanting to use the old lenses that they have (or can get cheaply) on their new body. Furthermore, how much would someone be willing to spend on an adapter when you can get an entire MD lens for $20?
