Impossible, right? That's what I said too when I ran across Metabones' new Speed Booster today (currently only available for pre-order). One important thing to get out of the way first off though before you get as excited as I did... this adapter is currently only available to convert from EF, ALPA and Leica-R mounts, to Sony NEX and Fuji X mirrorless cameras.
That said, lets get to the details. (If you want to check out the white paper all the juicy tech bits are in there)
The Speed Booster is similar to a teleconverter with one major difference. A teleconverter increases magnification where the Speed Booster (a focal reducer) has a magnification of x0.71. As a result more of the image, and more light is able to fall on the sensor. When DX and APS-C DSLR's were first introduced the question of making "inverse" teleconverters that reduced magnification rather than increased it was as well. The technology is in active use for telescopes, so why not a DSLR? Well, because of the reflex mirror. According to Metabones the mirror essentially "makes the design of a true general purpose focal reducer impossible." However now we have more and more "mirrorless" cameras and that changed the limitations. Now using the Speed Booster your 50mm f/1.2 will essentially become a 35mm f/0.90!
You're going to have to use full-frame lenses due to the focal reduction, but micro four-thirds Speed Boosters are in the works. Those sensors are small enough that using a DX or EF-S lens shouldn't be a problem.
The price points for these adapters are $399, $449 and $599 depending on the specific conversion you are looking for. So far Metabones only offers support for EF, ALPA and Leica-R mounts however according to their site:
"Other mount combinations will follow shortly afterwards. Contarex, Contax C/Y and Nikon F (with aperture control for G lenses) will be supported, as will Micro 4/3 and Fuji X-mount cameras. Support for other mounts will be added in the future."
I don't know about you but I'm pretty excited by the ability to play around at f/1.0 and below.