Just thinking of getting sigma 17-50 or older 18-50 f2.8 and found that the SA mount for sigma seems to sell for quite a bit less than the canon or Nikon versions. Question is would it be wise to stack adapters like SA to EOS then EOS to some speedbooster .71x?? Think the quality would suffer vs the native EOS mount on the speedbooster? Thanks
In theory, yes. In practice, it will probably be just fine.
Every mount interface suffers from some imprecision in the distance from the film plane or the angle relative to the film plane. If you use multiple adapters, the imprecision adds up. The speedbooster has a backfocus adjustment, so you can compensate for distance errors. Angle errors are usually only an issue for very wide lenses and when you are focusing on flat fields perpendicular to the camera.
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