Citation :

Petersburg, VA, USA, Aug 30, 2017: Caldwell Photographic Inc. and Metabones is pleased to announce the Speed Booster Q666, a.k.a. “The Devil’s Speed Booster”, exclusively for Pentax Q series cameras. With its 0.5x magnification and world-record f/0.666 maximum speed it is crazy wicked fast. So make a deal with the devil today and turn your Pentax Q with its back-illuminated sensor into a monster low-light machine unlike anything ever seen in photography. Attach an f/1.2 lens and stop down a little to see how Stanley Kubrick made do with f/0.7 for filming those famous candle-lit scenes in “Barry Lyndon”. Or open up all the way to f/0.666 and probe the darkness in a way that no photographer has ever been able to do until now.
With an advanced 6-element optical design, the Q666 packs lots of optical horsepower in a small package. Sharpness is excellent even at f/0.666, and is downright superb if you stop down to f/1.0. Distortion is very low, with a maximum value of, naturally, 0.666%. And thanks to the back-illuminated sensor used in Pentax Q cameras, the full f/0.666 maximum aperture can be utilized with minimal pixel vignetting effects commonly seen in other cameras used with high-speed lenses.
Figures 2 through 4 below show MTF at 10, 20, and 40 lp/mm as a function of image height for output apertures of f/0.666, f/1.0, and f/2.01. At f/0.666 (i.e., with an f/1.2 master lens) the MTF is high near the axis, and only falls off gradually toward the corners. As the aperture is reduced to f/1.0 and then f/2.0 the MTF becomes very high and more even across the field. Figure 5 shows that there is less than 1 stop of corner illumination falloff even at f/0.666. And guess what - there is no vignetting at all after the output aperture reaches f/1.82. Figure 6 shows that rectilinear distortion added by the Speed Booster Q666 is negligible at less than 0.666%.
Like all Metabones Speed Boosters, the Speed Booster Q666 is optimized to fully account for the camera’s filter stack located near the sensor surface. This is especially critical at the record-setting f/0.666 aperture now possible with the Q666. As a result, an enormous range of optics, from vintage film lenses to the latest digital designs, will function flawlessly when adapted to any Pentax Q camera. Planned lens mounts for the Speed Booster Q666 include a Pentax-K version and a Nikon F/G version with the most advanced Nikon G aperture adjustment mechanism in the industry.
The Speed Booster Q666 will be available starting in Aug 2017 from the Metabones website (http://www.metabones.com/products/details/MB_SPNFG-Q-BM1) and its worldwide dealer network for US $489.00, plus shipping and applicable taxes and duties.

Donc il s'agit d'un réducteur de focale de 0,5x qui augmente également l'ouverture maximale.
L'ouverture maxi est de F/0.666 pour une utilisation avec un objectif ouvrant à F/1 (si vous arrivez à trouver un objectif ouvrant à F/1...)
Vu le prix je ne sais pas s'il y aura beaucoup d'amateurs, pour bien moins cher on trouve des objectifs lumineux en monture C.