Metabones has just announced its
Speed BoosterTM mount adapter.
This interesting gadget lets you mount a full-frame lens on a mirrorless camera with APS-C or smaller sensor. It then acts as a sort of reverse teleconverter, reducing the focal length of the lens, while
increasing the size of the aperture.
There seems to be some confusion on the forums about this, but it's nothing magical - all it does is reduce the full-frame image produced by the lens by a factor of about 0.71, so the new, smaller image circle still covers the smaller sensor, but with (ideally) no cropping. Thus the lens with adapter behaves like a shorter (by a factor of 0.71) lens for crop-factor sensors. The total amount of light transmitted is still the same, but concentrated on an area that is 0.71
2 = 0.5 times the original area, thus the light intensity is doubled, giving you an extra stop of light hitting each pixel. Very cool! I wish I had thought of (and patented) this!