Film-style shooting, whether for a full-length movie or television drama, carries a different set of needs than employed for sports and other live news productions. Specifically, you need the right lens that matches your shooting style and your camera.
There are two types of digital cameras. The first uses a prism beam splitter with 2/3in 3-CCDs and a B4 mount. Examples include the Panasonic VariCam; Sony HDW-F900, HDC-F950 and F23; and the Grass Valley Viper.
The second type employs a single Super 35mm size sensor and traditional film lenses with an ARRI PL mount. Examples include ARRI D20, Red One, Dalsa Origin and Panavision Genesis.
The move to HD presents an interesting question: Do you base designs on SD television lenses or 35mm film lenses? You can take either route depending on the application.
Television is ephemeral, especially live, which is shot on the fly and aired. For videographic shooting, a television camera operator controls frame, focus and zoom. Television-style shooting is often at small apertures with a large depth of field so that the entire scene is in focus.
Television lenses are typified by the box lenses used for studio and OBs, or the lightweight zooms used for ENG and field production. Studio and OB box lenses have been developed to suit the demands of HD, similar to the ENG lens. Although eminently fit for purpose and built to a price, they do not provide the performance of a prime film lens.
The product of cinematography is longer lasting. A movie, television drama or high-end commercial is a carefully crafted product that is shown over and over again. A movie is displayed at a large magnification on a cinema screen. Cinematographic shooting is a controlled process where each shot is rehearsed. Lens focus is controlled by the focus puller or first assistant cameraman.
Whether full frame or 2/3in, digital cinematography has high-quality requirements for the optics. These go through considerable post production and often use green screen shots, requiring low chromatic aberration (color fringing).
Film style shooting can use differential focus so the viewer concentrates on one aspect of the scene. Differential focus needs a small depth of field.
For television, the zoom is ubiquitous and delivers convenience and speed of operation. Sports coverage would not be possible without the zoom.
For film, zooms are used only in special circumstances, where the sacrifice in image quality is offset by the requirements of the shot. A film lens is designed to provide the highest image quality that can be magnified without showing visible artifacts on a typical cinema screen.
The fixed focal length film lens, or prime, can deliver a higher quality, without all the compromises that are necessary in a zoom lens. A prime lens is simpler and has fewer elements than a zoom. It's smaller and lighter, making the camera easier for the cinematographer to use in confined spaces, such as inside a vehicle. The lower weight is an advantage when used on a handheld or a Steadicam camcorder.
Depth of field
If you do the math, depth of field is inversely proportional to image size for the same f-stop. The Super 35 diagonal is 31mm, as opposed to 11mm for a typical 2/3in sensor. That means for a given depth of field, the smaller sensor will need a wider aperture. Typical film apertures are f3.5 to f4. For a 2/3in sensor, this equates to around f1.6. The beam splitter within the camera body using the B4 mount has a maximum aperture of f1.4.
For film-style differential focus, HD lenses should provide optimum performance at wide open apertures. This creates challenges for lens designers. Conventional lenses perform best at mid-aperture settings, but HD lenses need to perform well at wide apertures. The lens performance should be limited by diffraction rather than common lens aberrations.
In 3-CCD cameras, the sensors are cemented to the beam splitter prism. Therefore, it is not possible to adjust the back focus within the camera body. To get around this lack of adjustment, camera manufacturers have agreed to a standard for the rear focal length. EBU Tech 3294 specifies the glass to be used in the prism, dimensions and filters. This standard allows lenses to be manufactured to suite these specifications and ensures interchangeability between camera bodies.
The focal length is different for R, G and B. For example, with 2/3in HDTV cameras, red is +10µm relative to green, and blue is +5µm. This different path length adds to the problems of correcting chromatic aberrations, therefore requiring more care in lens design to minimize these aberrations.
The long back focus means that primes have to be designed to be telecentric on the image size, or that the chief rays are parallel. This prevents color fringes from being created within the prism. This is not necessary with 35mm lens and adds to the cost of HDTV lenses.
Lenses are largely supplied through the rental market, so they have to be robust and be able to stand up to wear and tear. Yet at the same time, they are precision optical instruments, so the mechanical aspects of design and manufacture must be to the highest standards. This adds to the cost.
A prime lens will be more expensive than a mass-produced ENG lens. That said, all lenses are constrained by price, so the manufacturers of primes have to apply clever design techniques to arrive at an affordable product. Savings can be achieved by sharing components across a range of focal lengths. For example, the rear group and iris can be common to a product range, with the front group varying for each focal length.
Although lens design is constrained by cost, it ultimately depends on basic physics. If you put a 1/3in sensor in an HD camera, you are not going to realize a shallow depth of field.
The 2/3in sensor provides a good compromise. Affordable camera bodies can be produced, and the 3-CCD configuration offers better control over colorimetry than the single CCD cameras. The 2/3in primes provide the ergonomic features required by cinematographers, yet the same camera bodies fitted with a box zoom can be used for conventional television production, providing the best of both worlds.
This feature was written with assistance from Michael Bravin of Band Pro.