In a nutshell: Use your Canon lenses with your Sony E APS-C Body camera (Like the A7s series, or the FS5/FS7 cameras). I use it with my Sony PXW-FS5 and it's invaluable to have because of the reduced crop factor and extra stop it gives you; great in low light conditions. This speed booster is brand spankin' new so please take good care of it!

Canon EF Lens to Sony E APS-C Body
Full-Frame Lens to Sony E APS-C Body
Supports In-Camera Aperture Control
0.71x APS-C Crop Factor Compensation
1-Stop Increase in Lens Apertures

The T Speed Booster Ultra 0.71x Adapter for Canon Full-Frame EF-Mount Lens to Sony E-Mount APS-C Camera from Metabones allows a full-frame Canon EF-mount lens to be mounted onto a Sony E-mount camera. While doing so, the adapter's optics help to maximize the connected lens's apparent angle of view while providing a 1-stop increase for the camera over the lens's aperture range.
If a lens intended for 35mm film or similarly sized full-frame camera sensors is connected to a camera with a Sony E-mount APS-C-sized sensor, it would project an image larger than the Sony E-mount APS-C sensor was built to accept. Able to capture only the center portion of the projected image, the effective angle of view of the lens would be narrowed, creating an in-camera "crop factor." The T Speed Booster Ultra 0.71x Adapter for Canon Full-Frame EF-Mount Lens to Sony E-Mount APS-C Camera condenses the light from a lens designed for 35mm film or "full frame" camera sensors and projects it onto a Sony E-mount APS-C sensor. This allows a Sony E-mount APS-C sensor to utilize more of the light gathered by a connected lens, which effectively reduces the crop factor while making available to the sensor an additional 1 stop of light for each of the lens's f/stop settings.
This adapter is constructed from aluminum alloy for durability while minimizing weight and its mounting rings are made with brass to further prevent wear-and-tear. Inside are five lens elements in four groups that are designed to provide optical clarity as well as color fidelity, and a detachable tripod foot is included that features 3/8" threads as well as compatibility with Arca-Swiss tripod heads.
The T Speed Booster Ultra 0.71x Adapter for Canon Full-Frame EF-Mount Lens to Sony E-Mount APS-C Camera supports electronic communication between an attached camera and lens. If the lens includes autofocus, image stabilization, as well as camera-controlled aperture functions, this adapter is intended to allow those features to be used. If an attached lens is also capable of generating EXIF metadata when used with a directly compatible camera, the T Speed Booster Ultra 0.71x Adapter for Canon Full-Frame EF-Mount Lens to Sony E-Mount APS-C Camera is designed to allow that information to be saved along with image files generated with a combination of the lens, a Sony E-mount camera, and the adapter.
By enabling camera-controlled aperture functions for a Canon EF-Mount Lens when used with a Sony E-mount camera, program, shutter-priority, aperture-priority, as well as manual modes can be set and used through the camera. Power for these functions is transferred from the camera to the lens, and the T Speed Booster Ultra 0.71x Adapter for Canon Full-Frame EF-Mount Lens to Sony E-Mount APS-C Camera is designed to accomplish this in an energy-efficient manner. When manual focusing is preferred with a compatible electronic lens, an integrated switch can be depressed which opens the lens's aperture diaphragm to it largest setting. The resulting shallow depth-of-field can then be used to help ensure focus accuracy.
While the T Speed Booster Ultra 0.71x Adapter for Canon Full-Frame EF-Mount Lens to Sony E-Mount APS-C Camera is intended for Sony E-mount cameras with APS-C sized sensors, it may also be placed on "full frame" Sony E-mount cameras as well. However, such cameras must be set to "crop mode" when combined with this adapter to avoid generating images with vignetting.
Allows a full-frame Canon EF-mount lens to be used with a Sony E-mount APS-C camera
Maximizes apparent angle of view of combined camera and lens
Provides a 1-stop increase for the camera over the lens's aperture range
Five lens elements in four groups for optical clarity as well as color fidelity
Supports electronic communication between camera and lens to enable autofocus, image stabilization, on-camera aperture control, and EXIF metadata generation when possible
Internal coating minimizes reflections within the adapter barrel
Detachable, Arca-Swiss compatible tripod foot with 3/8" threads
Aluminum alloy components for weight reduction
Brass mounting rings prevent wear-and-tear
Locking mechanism for secure attachment of lens with push-button release