Optical
Increase maximum aperture by 11/3 stop
Increase MTF
Makes lens 0.64x wider
Patented 6-element/4-group optical design by Caldwell Photographic in the USA
Supports 4k video mode with DX lenses (except on GH5).
Electronic
Autofocus
Can be powered either by camera body or by third-party external 5V power source (not included)
Iris/aperture controlled by the camera body
Smooth iris support with the latest Canon (2009+), Tamron (SP series 2013+) and Sigma (2016+) lenses
Image stabilization (IS) lens support
Support for cameras with in-body image stabilization (IBIS), including Panasonic GX7, GX8, GX80/85 and Olympus cameras
Electronic manual focusing (e.g. EF 85/1.2L II and discontinued EF 50/1.0L)
EXIF (focal length, aperture, zoom range)
Distance and zoom display on camcorders (requires lenses supporting distance information)
Canon Cinema EOS lens (CN-E Compact-Servo) support, including AF (requires external 5V power through micro USB port), auto iris and servo zoom
Other
Detachable tripod foot compatible with Arca Swiss, Markins and Photo Clam ball heads.
The opening is flocked with felt material to reduce internal reflection.