https://www.metabones.com/products/details/MB_SPEF-m43-BT3

A new Micro-4/3 mount Speed Booster^® adapter designed exclusively for the Panasonic GH4 and other selected Micro Four Thirds cameras (refer to the below camera compatibility table), with a magnification of 0.64x.  The Speed Booster XL 0.64x uses an advanced 6-element optical design to achieve extraordinary optical performance at apertures up to an incredible f/0.80, a new record for Micro Four Thirds format.

The new Speed Booster XL 0.64x reduces the full-frame crop factor of the Panasonic GH4 from 2.0x to 1.28x, thus effectively transforming these cameras into APS-H format.  When the GH4 is used in Cinema 4k video mode the horizontal full frame crop factor is reduced from 2.34x to 1.50x, thus effectively transforming the GH4 into a super-35 format 4k cine camera.  In addition, the speed of any attached lens is increased by 1¹/₃ stops, with a maximum output aperture of f/0.80 when an f/1.2 lens is used.  For example, a 50mm f/1.2 becomes a 32mm f/0.80, which is the fastest aperture available for full size Micro Four Thirds format. (More information can be found in the press release here )

GH5, XL or ULTRA?

In a nutshell, we recommend XL for full frame lenses and ULTRA for DX/APS-C lenses. For the long story, read the ‘limitations’ column.

FEATURES AND BENEFITS

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.

LIMITATIONS

• Supports only Panasonic GH5s, GH5, GH4, GH3, G3, G5, G6, G7, G9, G10, GF3, GF5, GF6, GX1, GX8, GX80/GX85, GX9, Olympus E-PL7, PEN-F, Blackmagic Cinema Camera with Micro Four Thirds mount**, Pocket Cinema Camera*** and Micro Cinema Camera***
• EF-S lenses require modification to fit or may remain incompatible even after modification (but third-party DX lenses can be used without modification)
• On GH5, Speed Booster XL can be used with full frame lenses only. DX lenses are unable to cover full-sensor-width readout + in-body image stabilizer (IBIS)
• EF-S and third-party DX lenses do not cover the full Micro Four Thirds sensor in still photo and FHD video (but will cover 4k video mode on GH4, GX8 and GX80/85).
• No video AF for Olympus
• AF may not work if the maximum aperture of the lens plus any EF Extender or other teleconverter attached is smaller than f/8 before boost or smaller than f/5.1 after boost.
• AFC (continuous AF) and AFF (flexible AF) modes are not supported, except on Olympus OM-D E-M1.
• Panasonic video AF may hunt more than a native lens or have unsatisfactory performance.
• Some Tamron lenses may have unsatisfactory AF accuracy.
• No support for Focus Stacking and Focus Bracketing on Olympus.
• On Olympus, other than E-M1, you may move the single AF point among any of the 81, but you may not multi-point AF. There is no restriction on E-M1 and Panasonic.
• Very early Olympus models (e.g. E-PL1) have no autofocus support.
• EF-mount lenses are not silent but may make audible click noises during autofocus operation and iris changes which may be picked up by the built-in microphone of the camera. An external microphone is recommended for video.
• Use of Canon Cinema Compact-Servo lenses such as CN-E 18-80mm T4.4 L KAS S for still photography is not supported.
• Iris changes are in 1/8 stop steps which may be noticeable in video recordings. Programmed exposure mode and shutter priority exposure mode should not be used for video. This limitation does not apply to Cinema EOS lenses which permit fine resolution iris control, such as CN-E 18-80mm T4.4 L IS KAS S.
• No support for in-camera correction such as peripheral shading, CA and distortion