Manual focus lens adaptors for AF cameras
Is it possible to get an adaptor which will allow such a lens to focus to infinity without any optics?
Depends on the mounts involved; M42 can mount on almost anything with infinity focus, Leica M cannot on any SLR (though it can on most mirrorless systems), and so on. Google is often your friend in this, but you can always ask, as well.
Either way, can anyone recommend a good brand?
Again, depends on the mounts involved. Though I've heard Fotodiox are pretty good, for the brands they work with.
Generally you need at least a millimeter of register distance for an adapter. There are some exceptions when the lens is close and originally for a smaller mount, though even that only gets a small amount because the mirror provides a hard limit.
Here's a page with a good start for listings. http://en.wikipedia.org/wiki/Flange_focal_distance
Unfortunately MD (called SR in the list) is shorter than your Sony A mount (and just about all DSLRs), so it's very unlikely that you'll find a glassless infinity adapter. MD mount (and Canon FD mount and Konica mount) lenses are typically only adaptable to mirrorless bodies which have very short register distances.
Generally any adapter works fine, cheap ones are made from aluminum and cut shorter than necessary (ie for an adapter that needs to be say 1.35mm thick it will be 1.338-1.342mm thick) to ensure that all lenses will reach infinity focus on all bodies. This doesn't matter for most prime lenses where the entire optical system shifts as a unit, but can reduce performance on lenses that internally focus or focus from the rear (typically zooms or rarely primes). If the adapter is made from two pieces a small amount of aluminum foil can bring the lens to the correct length for one's camera and lens. This is a bigger factor for video work as it affects lenses that are parfocal. Fotodox and Rainbow Imaging are common sellers of cheap adapters, they can be found on ebay or amazon (and likely other sellers) for many possible mounts.
Better adapters are made from brass which reduces the risk the adapter will experience galvanic corrosion (a rare condition where dissimilar metals bond), to tighter tolerances (which may reduce the thickness problem I mentioned earlier. Because of that issue I would not expect most adapters to work with things like the metabones speedbooster unless they were very carefully adjusted.
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