The B4 lens mount was standardized in 1992 by the Broadcasting Technology Association (BTA) and is defined in BTA S-1005.^[1] This standard defines the physical mount, but also optical properties and some electrical connections. The B4 mount defines the sensor to have a diagonal size of 11 mm (a so-called 2/3" size sensor). The B4-mount is used by practically all 2/3" broadcast lenses and cameras (as of 2019).

Although the standard was set in 1992, the B4 mount already existed before 1980. The Sony BVP-300, produced from 1978, was possibly the first camera with a B4 mount. Further, all Sony Betacam cameras had a B4 mount.^[2]

The BTA was formed by Japanese broadcaster NHK and included members from Canon, Fuji, Hitachi, Ikegami, JVC, Matsushita (Panasonic), Nikon, Sony and Toshiba. It was formed in the mid-1980s and set various standards for television. It is now part of ARIB, Association of Radio Industries and Businesses.^[3][4]

The flange of the mount defines the positioning of the lens relative to the image sensor. A ring is present around the opening on the camera which, when rotated, tightly locks the flange of the lens against the camera. A pin on the top side of the lens flange and a hole in the camera mount make sure the lens cannot be mounted at an angle.^[5]

The B4-mount has its image projected at 48 mm behind the lens mount flange (in air). The standard defines that a prism splits the light to form separate images planes for the colours red, green and blue. A correction for chromatic aberration is also part of the standard: the red sensor should be 10 μm further, and the blue one 5 μm further than 48 mm. This fitted well with established TV-camera technology using 3 tubes, and also with 3CCD, a technology in development at the time of the definition of the standard.^[6]

The standard defines that the diagonal size of the projected image should be 11 mm, but does not define a resolution to be used. In the past standard definition was captured at 4:3 aspect ratio. Nowadays, cameras with the same mount capture HD with 16:9 aspect ratio, or even 4K video, thanks to improved lenses.^[7]

The lens mount is accompanied by a connector for the electrical connections. It powers the lens motors, controls the iris and allows a few buttons on the lens handle to control camera functions. The connectors are made by Hirose. On the camera is a HR10-10R-12S receptacle, and on the lens a cable with a HR10-10P-12P plug.^[8]

The digital communication on pins 11 and 12 of the Hirose Connector is based on the Fujinon L10 Protocol,^[9] which was originally developed for external Control of their Digipower line of lenses, as well as their CCTV Line (called C10 there).^[10] The communication is based around inverted UART at 0v-5v TTL levels. However, the Protocol got expanded quite a bit since the last Publicly available revision V1.4. Known Differences between the L10 protocol, and the protocol actually used between lens and camera include:

• Autobauding: The lens has to be capable of automatically detecting the baudrate of the camera, since different cameras use different baudrates. The Blackmagic Design Ursa line uses 78400Boud for example.
• Undocumented features like request for the Lenses Serial number, more "Switch" Position feedbacks and additional set and request commands for Analog values.
• The request for multiple feedback values at once got expanded to 3 commands with up to four values each per request (five values according to reverse engineering)^[11]