Metabones MFT>Canon EF T Speed Booster XL 0.64x (by far the sharpest of all speedboosters out there, with a f1.3 gain in light and 0.64x in minimization for S35 and full frame lenses).
Make sure your lens is compatible with this booster and the camera/sensor-combination you’re looking to use.

+ Canon EF Lens to Micro 4/3 Body
+ Full-Frame Lens to Micro 4/3 Camera Body
+ Supports In-Camera Aperture Control
+ 0.64x Micro 4/3 Crop Factor Compensation

The T Speed Booster XL 0.64x Adapter for Full-Frame Canon EF-Mount Lens to Select Micro Four Thirds-Mount Cameras from Metabones allows a full-frame Canon EF-mount lens to be mounted onto a Micro Four Thirds-mount camera. While doing so, the adapter’s optics help to maximize the connected lens’s apparent angle of view while providing a 1.3-stop increase for the camera over the lens’s aperture range.

This adapter can be mounted onto the Panasonic DMC-GH4, GH3, G3, G5, G6, G10, GF3, GF5, GF6, and GX1 Mirrorless Micro Four Thirds Digital Cameras; the Olympus PEN E-PL7 Mirrorless Micro Four Thirds Digital Camera; the Blackmagic Cinema Camera with Micro Four Thirds mount; and the Blackmagic Pocket Cinema Camera.

+ Allows a full-frame Canon EF-mount lens to be used with a Micro Four Thirds-mount camera
+ Maximizes apparent angle of view of combined camera and lens
+ Provides a 1.3-stop increase for the camera over the lens’s aperture range
+ Integrated jog dial for incremental aperture adjustment and push-button access to lens’s widest aperture setting when used with a compatible Panasonic camera
+ Six 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