Fast lenses are a hot commodity in any photographer's gear bag, but what if you could make a lens faster than it already is? The Metabones Speedbooster seems to have screwed with optical physics enough to make this a possibility...for mirrorless cameras at least. They claim that the adapter "increases maximum aperture by 1 stop", but is this all a bunch a sly marketing?
What might seem like dark photo magic is actually a clever tele-compressor that allows you to take advantage of more light between a full frame lens and mirrorless camera. Great, but what does that mean? It means that if you put a full frame lens on a mirrorless camera, the adapter decreases the image circle enough to allow more light onto a smaller space. For example: A full frame 300mm lens on a NEX body would usually have an equivalent focal length of 450mm, but a telecompressor would make the lens appear wider, eliminating crop factor and lost light.
Metabones claims that the adapter also increases sharpness...in theory it will make better use of lens' original resolution, but this won't make the lens itself sharper. An important thing to remember is that adding another lens element could make it fall victim of additional chromatic aberration. Also, the price is pretty steep for the little converter: $600 for the EF to NEX mount and support for Alpha or Leica lenses are around the same price range. The big question: Is it worth the high price for an additional stop of light or would you rather just have a full frame camera?