Back in 2001 I wrote an article titled “Can you use Minolta manual focus lenses on an autofocus body?” That was before Minolta released their first digital SLR, and before they sold their camera business to Sony. The information, however, is still valid, and applies to the Sony Alpha digital SLR cameras for the same reasons. The short answer is, “No, you can’t use older Minolta manual focus lenses on a Sony Alpha DSLR camera”. They are different mounts, and would require an adapter. That’s the bad news. The good news is an adaptor exists. However, it has a piece of glass in it, kind of like a teleconverter, and this is a problem.
Why does the adapter have glass? The MC/MD and AF mount rings are different sizes (the MC/MD ring is about 30% smaller). So one side of the adapter accepts the MC/MD lens, the other mounts to the AF body. Now any adapter that can do this, purely from the mechanics, is of necessity several millimeters too thick. Without correcting optics, it would become in effect a short extension tube. And why is this a problem? If you’ve ever used extension tubes (which are great for their intended purpose!) you would notice that when the lens gets further away from the camera, the focal plane gets closer to the front of the lens, but the lens will no longer focus to infinity (the depth of field gets narrower too, and often curves). Now this is just what you want for macro photography, but not for much else. So to allow the Minolta manual focus MC/MD lenses to focus to infinity, a pieces of glass is necessary to correct the for the problem of the lens being slightly too far away from the film plane (or digital sensor plane). In such adapters, this piece of glass is usually about 2cm in diameter (not ideal for large aperture lenses in the first place) and is also not of very high quality (reasonable quality, to be sure, but not high).
The result is that when you mount your expensive lenses on this $40 adaptor, they all become considerably worse (I’ve done tests with an MC 58mm f/1.4, an MD 28mm f/2.8 and a Rokkor-X 135 f/2.8). The resulting pictures look pretty bad when compared to these same lenses on the intended camera body (a Minolta manual focus body like the X-700). Put in absolute terms, the results were worse than the cheapest 28-200 AF zoom in every area except distortion. That is, sharpness and contrast suffered noticeably. In digital cameras, you’ll notice this even on a six megapixel body. On a film camera, if you never enlarge your pictures beyond 3×5″ (9×13 cm) then you may not notice. Likewise, if you have never developed an eye for variations in contrast between lenses, and can’t see the practical difference between a $100 zoom and an $800 fixed focus lens, then again, you won’t have any problem with the results. In my test, a Sigma UC III 28-105 AF zoom (about $150 new, today you can get them on eBay for $50) gave instantly visually better results than the best lenses through the adaptor, and on 3×5″ prints! No magnifying glasses, no getting really close. Lay them all out on the table, and you can pick out the ones from the adaptor vs. the ones from the Sigma consumer zoom from a distance.
The pictures through the adapter are still better than most plastic lens point&shoot film cameras, but that is not a very good reference point. Of course, the MC/MD lenses are wonderful on an SRT cameras (and better than the Sigma zoom). So was it worth it to get a $40 adapter from China so that you can use high-quality Minolta manual lenses on a Sony alpha digital camera? To me the answer is clearly no. While it is technically possible, the results do not justify it in the least. Even a used $40 basic consumer zoom lens will produce better pictures than a top-quality prime through these adapters.