The Nikon G & F Lens to Micro Four Thirds Mount Camera Speed Booster from Metabones allows a full frame Nikon G & F lens to be mounted to a Micro Four Thirds mount camera and in doing so, increases the angle of view and increases the maximum aperture one f/stop.
When a lens intended for full frame sensors or 35mm film is mounted to a camera with an Micro Four Thirds sized sensor, it transmits a larger area of light than the smaller sensor was built to accept and therefore the angle of view of the lens decreases and the equivalent focal length increases. The Speed Booster condenses light from the lens and projects it onto the smaller sensor allowing the sensor to utilize all of the lens' light and effectively produce a full-frame angle of view with an MFT sensor. A corresponding increase in the maximum aperture of one f/stop makes the lens more effective in low-light situations and able to create a shallow depth of field.
The Nikkor 2.1cm f/4 and 20mm f/2.8 AI-S lenses are not compatible with the speed booster.