First, note that the basic idea is not new: it has been used in astronomy for a long time, under the names like Barlow lens or focal reducer. The key is that lenses like telescopes that are designed for far narrower than "normal" angular field of view [FOV] still naturally cover a roughly normal angle of about 40° to 50°, just producing an image far larger than necessary, so that the image then gets cropped by the sensor, or perhaps by in-lens anti-flare baffles or such. So telescopes can easily be adapted to produce a brighter image of a wider FOV when that is wanted.
Also, Nikon and Fujifilm used this idea in some early DSLRs, the Nikon E2/Fujix DS-560 etc., which combine a 2/3” sensor with a built-in 4x focal reducer and then use 36x24mm format lenses with their uncropped FOV (and roughly equal DOF, and low light abilities comparable to using the same sensor technology in 36x24mm format, thanks to the 4 stop increase in brightness given by the convertor.)
http://en.wikipedia.org/wiki/Nikon_E_seriesBut as far as abberations and such, the result is no better than a lens that is well-designed from the start for the shorter focal length and lower minimum aperture ratio in the smaller format. The main reasons that focal reducers are being offered now for the new mirrorless digital systems, but were not offered previously for SLR's, are that:
1. There is a great variety of existing lenses for 36x24mm format in particular that provide an image circle big enough for this image compression to work without vignetting with the new smaller formats. Similarly for "APS-C" lenses adaptable to 4/3” format.
2. The shallower lens mounts of the mirrorless systems provides room for the convertor.
3. Some adapted lenses can provide lower aperture ratios than are _currently_ offered by any "native" lens for the new smaller formats, like converting an f/2.8 zoom lens to an f/2, or a f/1.4 normal prime for one format to an f/1 normal prime for a smaller format. (But this reflects only a current lack, not a fundamental limitation of the smaller format. For example, there are some f/2 zooms for 4/3” format, just not in MFT mount.)
4. Some people already own enough lenses for the older larger formats that the adaptor can be cheaper than buying equivalent lenses for the new smaller format. And maybe save weight if one carries two systems at once for some reason.
