A reverse telephoto adapter?

Metabones has just announced its Speed Booster^TM 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 0.71² = 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!



Example: A 50mm f/1.4 lens with this adapter behaves like a 35mm f/1.0 lens. The angle of view is still roughly the same, but the low-light capability is so much more impressive.

Another example: mount a Canon TS-E 24mm f/3.5L II lens, and you get a 17mm f/2.5 tilt-shift lens for a small mirrorless camera! Now that would be my dream lens - it would be absolutely amazing for night-scapes, for which the additional speed would be extremely valuable.

I've been dreaming of owning a full-frame digital camera for a long time, for the following reasons:

1. Larger viewfinder.
2. Better image quality - a full frame sensor has twice the area of an APS-C sensor, hence you can expect a one stop improvement in image noise and high-ISO performance - partcularly useful for nightscapes (which I have been concentrating on recently).
3. Tilt-shift lenses like the above-mentioned 24mm. Of course you can mount such lenses on an APS-C camera like my old 40D, but you waste a lot of the wonderful image circle on the crop factor.

Now with this sort of adapter, you can have (3) on something like the Sony NEX 7 or Fuji X-Pro, and since the larger aperture means you can use one stop lower ISO setting - note that DOF will still be the same as you'd see with the original lens on a full-frame body - you also get (2). See this interesting article on the equivalence between different sensor formats. Once electronic viewfinders mature, you'll even get (1). Maybe that's the route I'll go one day...

Moreover, since the image is reduced, the actual sharpness should increase (provided that the adapter itself doesn't introduce too many aberrations of its own).

For now, the adapter exists only for mounting Canon-EF lenses on Sony E-mount cameras (with various limitations on autofocus), but other variations are in the pipeline. Of course, this can only work on mirrorless cameras since the adapter takes up space between the lens and sensor which is only available thanks to the shorter flange-back distance of mirrorless cameras.

I must say, I'm quite excited!

For more technical details, read the white paper yourself.