Metabones and optics company Caldwell Photographics have jointly announced the ‘Speed Booster’, a lens adapter for for mounting SLR lenses on APS-C and Micro Four Thirds mirrorless cameras that reduces the focal length by a factor of 0.71x and increases the maximum aperture by 1 stop.
Metabones announces ‘Speed Booster’ lens adapter for mirrorless cameras: Digital Photography Review.
By the power of physics! I still think they sound a little silly for the price, but could make for some neat wide-exposure lens pairings.