The Speed Booster works by shrinking the image circle of your Canon EF lens down to better fit the smaller sensor of the Blackmagic Pocket Cinema Camera. This has the effect of making the field of view of the lens 0.58x wider or, in other terms, reducing the crop factor of the camera from 3.02x to 1.75x. For example, your Canon 85mm f/1.2L becomes an effective 49.3mm with this Speed Booster, which renders a view equivalent to 148.9mm in 35mm full frame terms. This is close to the APS-C view of 136mm for this particular lens example. Your Blackmagic Pocket Cinema Camera will read less like a Micro Four Thirds sensor and more like the larger APS-C sensor thanks to this Speed Booster, plus you have the benefit of using a wide variety of Canon EF lenses while retaining lens AF (from firmware v2.1) and image stabilization (see Notes tab for more info on lens compatibility).
By concentrating the image circle from a lens, this Speed Booster also "brightens" the image – maximum lens apertures are increased by 1 and 2/3rd stops. As an example, this yields a maximum aperture of f/0.74 when using a Canon 85mm f/1.2L. This Speed Booster is powered by the camera body, so no additional power supply is needed. To reduce the drain on your camera's battery, it uses an efficient switched-mode power supply, which continually switches between on and off states and only draws power when it needs it. The Metabones Canon EF Lens to Blackmagic Pocket Cinema Camera T Speed Booster is a great tool for those using the Blackmagic Pocket Cinema Camera who want to also use a large selection of Canon EF mount lenses while getting that added benefit of a "boost" to both field of view and maximum aperture.