Figure 1: B4 mount of a Fujinon A13X4.5 lens
For over 50 years, Hitachi Kokusai broadcasting and production camera systems have delivered high-performance image acquisition for demanding live and recorded events.
Let’s start at the beginning: A camera lens is a precision optical and mechanical device with multiple lens ‘elements.’ These ‘elements’ are pieces of shaped glass that ‘bend’ light in various ways. Each element has a slightly different function, and they all play nicely together in order to form a sharp picture on the camera’s image sensors.
In its simplest form, a camera lens must focus light beams onto the camera’s image sensors. There’s a lot of physics that goes into lens design. The biggest challenge is that the lens shouldn’t be sharp in just one part of the picture; ideally, it should be perfectly sharp across the entire image, all the way to the edges and corners.
Zoom lenses must focus light beams into very exact locations on the imaging sensor while also offering smooth, continuously changing focal lengths. This is not an easy thing to do. Today’s typical cost reductions in electronics don’t apply to the optical and mechanical nature of lenses. The cost of some large zoom lenses can be much more than the cost of the rest of camera system.
Professional system cameras almost always support interchangeable lenses, enabling customers to flexibly choose the lens – or multiple lenses that can be switched from project to project – that best meets their needs. Practically all professional broadcast cameras use the worldwide B4 standard to mount the lens to the camera head.
The History of the B4 Lens Mount
The B4 lens mount standard was established back in 1992 and defines the sensor size to be 2/3-inch (approximately 17mm), consistent with the outside diameter of a 2/3-inch camera tube. Due to the thickness of the tube’s glass, the actual size of the resulting image was only 11mm diagonal. That standard of 11mm image size has been carried over and continues today in virtually all 2/3” solid state image sensors.
Unlike adjustable pickup tube imagers, solid-state image sensors are glued onto color-separation prisms. To solve that problem, the industry organized discussions of a standardized lens-camera interface dealing with the physical mount, optical parameters, and electrical connections. Participants included both camera and lens manufacturers. Collectively they developed and standardized the B4 mount.
The mechanical 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.
There is much more to the B4 mount than just the mechanical connection. The B4-mount optical standard has its image projected at 48 mm behind the lens mount flange. It uses a prism that splits the light to form separate image planes for the colors red, green and blue, which fit well with established TV camera technology using three tubes, three CCDs, or three CMOS sensors. There are specifications of different focal planes for the three color sensors; types of glass to be used in the color-separation prism and optical filters; and electrical signal connections for iris, focus, zoom, and more.
The lens mount is accompanied by a connector for the electrical signals. It powers the lens motors, controls the iris, and allows a few buttons on the lens handle to control camera functions.
From SDTV to HDTV and Beyond
Figure 2: B4 lens mount on a Hitachi camera
The B4 standard defines the diagonal size of the projected image on the sensor but does not define a resolution to be used. In the past, standard definition video was captured at a 4:3 aspect ratio. Today, cameras with the same B4 mount capture high-definition video with a 16:9 aspect ratio – or even 4K/UHD video – thanks to improved lenses.
Over the years, seven different companies have manufactured lenses with the B4 mount. When HDTV began to replace standard definition cameras, existing 2/3-inch lenses could be used on the new HD cameras. Even though those existing lenses were standard-definition, the B4 mount enabled customers to start using new HD cameras with already-owned lenses until they had the budget to invest in new HD lenses. However, with older SD lenses, the “modulation transfer function” (MTF) – the amount of contrast that passes through the lens at different frequencies, or resolutions – would be compromised. The MTF is better in lenses intended for high resolution applications, and the higher the MTF, the sharper the pictures will look.
It shows the staying power of the B4 mount that camera manufacturers maintained the 2/3-inch prism format in cameras from SD to HD to 4K. This has allowed users to upgrade the camera without the cost of upgrading the lens at the same time.
Today’s B4 Lenses
Figure 3: 20X8.5 lens 20:1 zoom 8.5mm wide
Most B4-mount lenses today are zoom lenses. An extensive range of high-quality lenses with the B4 mount is available, and typically include manual or servo iris and manual or servo zoom, with a servo rocker on the lens. Most of them have a simple slide ring for easy back focus and many also have a slide ring for macro focus for closeup shots. A wide array of studio or EFP servo zoom and focus controls are also available. All of these capabilities are available on cameras that can be used on your shoulder, a tripod, a jib, a dolly, a Steadicam, or other camera support options.
All B4 lenses are par-focal, meaning they maintain focus while zooming. The best and most expensive sports box lenses have an extremely large zoom range. They are also fast, meaning the lens has a large maximum aperture with large elements to gather as much light as possible. (For example, f/2.8 is a very large aperture, while f/16 is a small aperture). These large sports lens models are available with either optical stabilization or auto-focus.
Figure 4: 4K Sports lens. 107:1 zoom, 8.4 wide, f4.5-5.6
Other camera types such as single-sensor, DSLR, or PTZ cameras do not directly use the B4 lens mount. There are about a dozen brands of adapters available for mounting a B4 lens to an EF or PL-mount single sensor camera, but even an optically perfect adapter would lose more than 2.5 stops, requiring about six times more light. Furthermore, because nothing is perfect, the adapter would introduce its own degradation and aberrations to the resulting images. And from a practical perspective, using a large-format long zoom lens with such an adapter is not realistic.
As such, multi-camera, live event production almost universally uses professional system cameras with a B4 lens mount and three 2/3” sensors mounted on a prism. All Hitachi broadcast and professional production cameras – including the Z-HD5000, Z-HD5500 and SK-HD1800 HDTV cameras, as well as the SK-UHD4000 4K camera – use the B4 mount standard to give users an extremely large selection of long zoom, wide angle and many other high-quality lenses to choose from.
