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]
| Pin | Function | Direction | Description | |
|---|---|---|---|---|
| 1 | RET SW | lens to camera | Connected to RET button on lens; GND when pressed, otherwise open | |
| 2 | VTR SW | lens to camera | Connected to VTR or REC button on lens; GND when pressed, otherwise open | |
| 3 | GND | camera to lens | Ground | |
| 4 | IRIS ENF AUTO | camera to lens | Momentary auto iris, off: 0V, on: 5V | |
| 5 | IRIS CONT | camera to lens | Voltage indicative of requested iris position, f/2.8: 6.2V, f/16: 3.4V | |
| 6 | +12V | camera to lens | Power supply | |
| 7 | IRIS FOLLOW | lens to camera | Voltage indicative of current iris position, f/2.8: 6.2V, f/16: 3.4V | |
| 8 | IRIS A/R | camera to lens | Iris auto: 0V, remote: 5V | |
| 9 | EXT ANS | lens to camera | Connected to GND when extender is engaged, otherwise open | |
| 10 | ZOOM FOLLOW | lens to camera | Voltage indicative of current zoom position, Wide: 2V, Tele: 7V | |
| 11 | RxD | lens to camera | Receive serial data (sometimes instead used for focus follow) | |
| 12 | TxD | camera to lens | Transmit serial data |