Carl Zeiss DigiPrime lenses from Band Pro are purpose-built for digital cinematography.
Lenses and their design and manufacturing can be a mysterious and daunting subject for most of us. We know when a lens is really bad because we see the bad images it makes, or we feel it's sloppy mechanical operation. But what is a good lens, and what are the real differences between a film lens and a video lens?
Traditional lenses for video
First of all, lenses are not designed for film or video per se. Lenses are designed to meet certain performance criteria specific to an image size and imaging system. The distinction is more on the application, so this article will use the term “cine” for lenses that shoot commercials, movies and dramas. It will use the term “video” for lenses that make video for television and general video applications. 35mm film systems have a large image size and a fixed single plane image area. Digital cameras such as the CineAlta (Sony HDW-F900), Varicam (Panasonic AJ-HDC27F) or Thomson Viper, which use a beam splitting prism with a CCD imager, require a correction for how light is changed as it bends its way through the beam-splitting prism on the way to the imager. Because the prism splits the light into its red, green and blue components and steers the blue light to the blue sensor, red light to the red sensor, etc., the lens needs to correct for what happens to the light as it goes through the prism. Film lenses do not have this design criteria.
Traditional lenses designed for video are well-designed for their purpose. They have proven themselves over many years in applications where the material is captured and shown in real time without any rehearsals and usually viewed only once. Applications such as news, sport or live entertainment shows are produced for the television screen, and the image quality of such a lens only needs to be sufficient for television broadcasting seen on a TV screen.
The focus scales do not need to be real accurate because focusing is designed to be accomplished with a single hand while manually focusing by eye in the viewfinder. Zooming is done by means of a servo motor drive, and the aperture is automatically controlled by the camera.
Because cost, size and weight are primary design concerns, most traditional lenses for video are not well-corrected for several problems encountered in lens design. For example, these lenses are designed with multi-element optics systems that are usually not well-corrected for flare, veiling glare or ghosting, which is caused by internal reflections and stray light bouncing around inside. This results in a loss of contrast, color saturation and narcissism, which is when the lens adds its own internal reflections to the image.
Traditional lenses for video also are not well-corrected for barrel and pincushion geometric distortion, a bowing or squeezing of the picture.
In addition, they exhibit lateral chromatic aberration when all of the colors of the spectrum do not focus on the exact same plane. When they do not, the slightly out of focus color causes a color fringe along edges in the picture seen often on practical lighting or sharp transitions from light to dark.
Other problems include:
- Breathing — a noticeable change in image height as the focus changes. For example, on a long focus pull, a subject may appear to get unnaturally smaller as it walks closer. Or, in a rack focus, the subject changes size as it comes in or out of focus.
- Port holing — visible light fall-off to the corners, due to insufficient diameter of optical/mechanical elements.
- Ramping or f-drop — an unwanted exposure variation with zooming due to insufficient size of lens front group.
The cine lens
While well-designed for their intended application, these lenses fall short in the areas of image quality, mechanical precision and robustness found in the traditional cine lens for film cameras.
The Carl Zeiss DigiZoom lens from Band Pro is a precision zoom lens developed to enhance the potential of high-definition cinematography cameras.
Cine lenses are mostly primes or zooms, with short zoom factors. They are mechanically designed to accurately and repeatably capture well-rehearsed performances. Their focus iris and zoom functions are manually operated or assisted by precision motors and controllers. They are designed to primarily live on a tripod, dolly, jib arm or crane, or to fly on a Steadicam.
Their focus scales and iris rings have a predefined torque to aid in accurate, repeatable manual operation, and the scales are calibrated for accurate and repeatable measurement and setting.
Flare and veiling glare are normally controlled to imperceptible levels by minimizing the number of lens elements, especially a low number of glass-to-air surfaces. This leads to better controlled flare and ghosting, producing crisper, more brilliant, vibrant images with vivid colors and saturated blacks not exhibiting narcissism.
Another characteristic of cine lenses is excellent chromatic correction with virtually no color fringing and with a telecentric optical design at the output side of the lens. This enables precise green screen work with no color fringes, easing the post-production process. Geometric distortion is limited to undetectable levels, and the lenses do not have perceptible breathing, port holing or ramping.
When designing a lens for cine applications, there is a different set of criteria addressed in the design than when designing a lens for video applications. Lenses for film have to produce images displayed on 50ft motion picture screens or images that contain large amounts of information as a the basis for extensive post-production work. They are primarily designed as a craftsman's tool to create the visual content and to support the emotional content and story telling in long-lasting, multimillion Euro productions.
Relays and adapters
In summary, lenses designed for video applications stress size, weight, automatic operation and price. Lenses designed for cine applications stress precision, optical and mechanical operation, and correction for distortion, artifacts and aberrations.
With the recent rise in high-definition cinematography, there has been a need for a cine design mentality applied to the design for a video camera system with the prism and CCD imagers. Several lens manufacturers have risen to the challenge of designing optics for these systems with varied and interesting results.
The simplest and least expensive solution is to adapt a cine lens for film to be used on a digital camera. This is accomplished with a relay system. The CLA35HD from Carl Zeiss and marketed by Angenieux was designed as an interim step. It allows you to mount a PL cine lens onto a digital camera. It corrects for the prism optics in the camera.
Another approach is the Pro 35 from P&S Tecknik. It combines a relay adapter with a ground glass. The image is projected on the ground glass, where it is captured by the relay and directed into the prism. The relay system works the same to adapt for the prism optics, but because of the ground glass projection, the depth of field characteristics are close to a 35mm film system, i.e. 2.5 stops shallower depth of field.
The advantage of a relay or adapter is the ability to recycle existing lenses and accessories from the extensive motion picture film lens business. This gives you a broad range of focal lengths and devices that mount to a PL lens mount. Also, the shallow depth of field characteristics of a 35mm lens is carried into the digital world, which inherently has deeper depth of field.
Retrofit or adapt
The disadvantages are additional size and weight. With the Pro 35, a disadvantage is noise because it has a spinning motor moving the ground glass. Likewise, on this unit, you are imaging a ground glass image, so the quality of the image is determined by the quality of the ground glass.
The CLA35HD inverts the image, and on some cameras, this requires inversion circuit or modification. This unit also provides no improvement in depth of field control, so its real advantage is recycling older lens inventory.
Another solution is to remanufacture or adapt a cine lens designed for film cameras. This is done by adapting the lens mount and changing the optics in the rear of the lens to correct for the prism optics in the digital cameras. Cooke and Angenieux have chosen this path, and Optex did this with 35mm still lenses early on.
The advantages here are that you do not have to design from scratch, with the associate capital investment and time factors, and these adapted lenses are already known for their design and performance in the film business.
The disadvantage is that you compromise image quality to maintain existing designs and manufacturing efficiencies. These lenses tend to perform well for digital cameras but not nearly as well as the purpose-built lenses.
The third and newest category are purpose-built lenses. These are lenses that were designed from scratch to fully optimize their design to maximize performance with the 2/3in CCD prism imagers found in modern state-of-the-art digital cameras.
Several companies have shipped lenses that are purpose-built for digital cinematography. These include Panavision, with its Digital Primo primes and zooms; Canon, with its Cine series of primes and zooms; Fujinon, with its “E” series cine zooms and primes; and most recently Carl Zeiss, with its introduction of DigiPrime and DigiZoom lines.
The next step
The advantage is visibly superior optical performance because the lenses are specifically designed for the imaging system and correct for most of the aberrations that digital systems present.
The disadvantage of these lenses is their cost. The manufacture of such a lens is nearly two to two-and-a-half times more complex and precise to ensure a pristine imaging on a small 2/3in target. The cost is around €20,000 for a prime and €30,000 to €60,000 for a zoom (except Panavision, which only rents). These lenses represent a significant financial investment.
Recently, several manufacturers have introduced prototypes of a “new” generation of cine camera with a digital imager instead of film. Dalsa, Arri and Panavision have shown cameras that use traditional cine film lenses. The jury is still out on the relative quality issues in using lenses designed for film cameras on a digital imaging camera.
Can a purpose-built lens for these systems be avoided? As these cameras come to market and are tested, there will be more information, and the need for a new generation of large-format digital optics can be properly evaluated.
Michael Bravin is the chief technology officer for Band Pro Film & Digital.