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That Sigma mount is so limited I can't see any adapters available for it...
Then obviously you simply have'nt been looking in the right places!...There are several types of M42-SA adapter available for the SA mount, two types of Nikon adapter (one for the SD9 and SD10 only, the other for all SA mount cameras via an M42-SA adapter), Pentax K mount lenses can fit straight on without needing an adapter (via a very simple but often non-reversable mod), M645-SA adapters can be made very simply with a M645 #1 extension ring, and fully reversable mount swap kits are available for the SD14, SD15 and SD1, to convert them to either C/Y, Olympus OM, Nikon F, Canon FL/FD and Leica R mounts.
> If I have SD1 with a Sigma lens vs. NEX-5N + a great lens like CV 125,
guess which image will be better.
Thats a silly statement...The NEX cannot hope to match the SD1's resolution, even when the SD1 is fitted with a lower quality lens than the NEX, so obviously, the Sigma will walk it.
Luckily though, there is no need to use Sigma lenses on the SD1, unless you simply must have AF...But Canon EF lenses can be easily converted to SA mount and they can AF on the SA mount too so again, there is really no need to use Sigma lenses, on any SA mount camera.