Cinema Zoom Lenses

http://www.bhphotovideo.com/indepth/video/hands-reviews/cinema-zoom-lenses?cm_mmc=EML-_-Holiday-Thurs-_-131121-_-Body_Indepth_Cinema-Zoom-Lenses

By Steven Gladstone

Published: Wednesday, November 20, 2013 - 11:16am

People have been shooting HD video with DSLR cameras packing still lenses for years—mostly with fixed focal length lenses, which are often referred to as "prime" lenses in the film industry. In general, zoom lenses made for still photos are a poor fit for cinema production. You can use them, as many low-budget indie productions do, but they lack many of the necessary features that are required for capturing high-quality cinema images.

These circumstances have sparked a fair amount of growth in prime lens design and production for the cinema market, driven by the popularity of HDSLRs and mirrorless cameras used for HD acquisition. Add to this the development of cameras such as the Blackmagic Cinema and Pocket cameras, the Canon C100, C300, C500, and digital cameras from RED and Arri, and you can see why there is a growing market for high-quality cinema style lenses—especially zooms.
Zoom lenses are far more complex to design and build than prime lenses, and until the digital cinema camera boom really began to reverberate in the production market, around 2010, there were basically two manufacturers of zoom lenses for film production: Angenieux and Cooke. This is because as far as 35mm film cameras go, there was never a large market, and it became saturated long before the HDSLRs showed up. This resulted in a few extremely expensive options for cinema-style zoom lenses. Looking to the broadcast video market, there are many good quality zoom lenses available; unfortunately, these lenses were all designed for a smaller imager, and work with a prism that splits the image for three sensors used in video production. Because of this, these lenses are unusable with a single-sensor camera (film or digital) without using a costly adapter.
Now, however, with many more available single-sensor digital cameras being used to shoot HD video (and beyond), Zeiss, Fujinon, and Canon are producing cinema-style zoom lenses to supply the growing market, giving the creative user far more options than before.

Still versus Cinema: The Basics

Still photo lenses designed to cover a full-frame sensor are often smaller, lighter, and much less expensive than a cinema-style lens, which is designed to cover a sensor half the size. However, while cinema-style zooms and still-photo zooms essentially perform the same functions, it really is more about how each lens is doing its job, and under what conditions it has to do the job. Still-photo lenses are designed for taking a single photo at a time, framing, focusing, and refocusing for every shot, if necessary. Most still-photo zoom lenses aren’t really zoom lenses at all, they are more correctly called varifocal lenses. You aren’t really zooming while shooting stills so much as changing focal lengths by using one lens instead of carrying around several lenses and having to physically change from one lens to another.
However, when you shoot for movies/video you are shooting a series of images that must run uninterrupted, or the illusion of motion is going to be broken. There are four major functions at which a zoom lens can fail, which can affect the image and your video most. These are: focus shift, focus breathing, aperture ramping, and zoom tracking. Cinema-style zoom lenses are built to overcome these issues, which accounts for their increased size, weight, and cost.

Focus Shift: Varifocal versus Parfocal

A cinema-style zoom lens has to hold focus throughout the entire focal-length range. This requires that the   lens be parfocal, as opposed to varifocal lenses, which don’t hold their focus throughout the focal length range, requiring you to refocus when you change focal lengths. However, it is a necessity when shooting any kind of moving images that use a zoom. You can’t stop an actor in mid-word, re-adjust the focus because you are doing a zoom-in, and then continue shooting. If the focus changes noticeably within a shot it becomes distracting to the audience and adversely affects the impact of your piece. Constructing a parfocal lens requires much more optical compensation and complex mechanisms in the lens than are required to make a varifocal lens.

Focus Breathing

Focus breathing is an optical effect that occurs as you rack (adjust) focus in and out from one subject to another. As you adjust focus, the image size changes slightly and the resulting image begins to look like it is breathing. All lenses exhibit focus breathing, it is just a matter of how much. This is often more visible in zoom lenses than in primes, and can be very distracting to an audience. The less breathing a lens exhibits, the better.

(Click image to see focus breathing in action)

Aperture Ramping

One thing to be aware of is that a certain amount of light is lost as it passes through the lens, more so with zoom lenses than with prime lenses. With zoom lenses, the light loss can become much greater at the long end of the focal-length range. This effect is undesirable, and is often found on consumer-quality video lenses and varifocal still lenses. To avoid this problem and have the aperture remain constant throughout the focal-length range, you either end up with a small lens with a slow minimum aperture, or a large, heavy lens that will maintain a consistent aperture throughout zoom range.

(Click image to see aperture ramping in action)

Zoom Tracking

It is very important that your zoom lens tracks straight, and what this means is that if you zoom in and center your frame on a target, when you zoom out the center of the target stays consistent and doesn’t drift.  It is difficult enough to pull off effective zooms on a static subject without having to constantly adjust for the lens not tracking correctly.

(Click image to see a sample of poor zoom tracking)

Zeiss Compact Zooms

Zeiss has released a 28-80mm T2.9, a 70-200mm T2.9 and announced a 15-30mm zoom. Overall, the features of these zooms are exciting. The lenses cover a full-frame 35mm sensor, allowing you to use them on cameras with any of the popular sensor sizes from full frame, APS-H, Super 35, Academy 35, APS-C, and MFT. The lenses are also true zoom (parfocal) lenses, holding focus throughout the entire zoom range. These lenses, as with the Zeiss Compact Primes.2, offer the choice of five interchangeable lens mounts: PL, Canon EF, Nikon F, Sony E, and MFT. This makes it simple to forego adapters if you want to mount the lens on cameras with different mounts, and because you are swapping out the mount, you don’t have to worry about slop and play that may be introduced by the adapter. You can swap mounts, and ensure that your flange focal distance stays rock solid, and your zoom lens will deliver in focus images.

The Zeiss Compact Zooms are 4K ready, meaning they resolve enough detail to provide a sharp image when used on a 4K sensor. These lenses have also been designed and built to match the color of the Zeiss CP.2 prime lenses. This provides for a consistent look in your image when intercutting between the CP.2 primes and the Compact Zoom lenses, providing that distinctive Zeiss look, with all your lenses. The two currently available zooms feature a consistent maximum T-stop of 2.9 across the entire zoom range, and an iris design that produces rounded, out-of-focus highlights, which are very popular in photography. The focus and zoom barrels feature built-in gear rings so they are ready to interface with a zoom motor and follow focus without any modification. The focus barrel has nearly 300 degrees of rotation, providing for precise and repeatable focus pulls.

Fujinon Cabrio Zooms

Fujinon presents a 19-90mm T 2.9, an 85-300mm T 2.9-4.0, and announced a 14-28mm PL-mounted zoom lens. The lenses feature a 31.5mm image circle for use with many single-sensor digital cameras. Reflecting its heritage as an ENG lens maker, Fujinon also brings a removable ENG-style handgrip with built-in zoom control to the lens. This is an uncommon feature to find on a cinema-style zoom lens, as most cinema-style zoom lenses use a separate zoom motor and control unit. This will make for a comfortable transition for ENG camera operators who are now working with digital cinema cameras, and may also find favor among documentary camera operators. It is a nice feature, and the fact that it is removable, leaving you with a standard functioning cinema zoom lens, certainly makes this lens a very flexible choice.

The 19-90mm and 85-300mm lenses feature a 2.8’ and 3.9’ minimum focusing distance, respectively, a macro function, and they maintain a consistent maximum T-stop of 2.9. The lenses incorporate a 9-blade iris that provides for round, out-of-focus highlights. Borrowing again from its ENG roots, Fujinon incorporates a built-in back focus adjustment, which means that you can adjust the lens to the camera in the field, without un-mounting the lens and collimating it on a bench. The PL mount also includes both the LDS and iTechnology electronic data system that allows the camera to record lens data such as iris information, focus, and focal-length information. The lenses feature 200 degrees of focus barrel rotation, which provides ample room for your focus puller to work.

Canon Zooms

Canon has released four cinema-style zoom lenses that are capable of 4K images, and cover Super35-sized sensors. The Canon lenses are separated into two groups: the zoom series, which includes a 14.5-60mm T 2.6 and 30-300mm T 2.95-3.7, and the compact zoom series, which includes a 15.5 to 47mm T 2.8, and a 30-105mm T 2.8 zoom lens. The lenses all feature common gear positions and rotation angles for zoom, focus and iris—this design makes for quick lens changes between the Canon cinema zooms. The lenses are available in either EF or PL mount, and while you can have the mounts swapped out, Canon recommends this be done at the Canon factory. As with most of its lenses, Canon uses Flourite glass and aspheric elements in their lenses, and this is consistent with the construction of Canon’s cinema prime lenses, for easy matching with footage shot on the Canon cinema primes.

The 15.5-47mm and 30-105mm both feature a consistent maximum aperture of T 2.8. The 14.5-60mm features a maximum aperture of T 2.6, while the 30-300mm features a maximum aperture of T 2.95 from 30 to 240mm, and T 3.7 at 300mm. The 11-bladed circular iris provides round, out-of-focus highlights, popular with users of modern lenses. The focus barrel features 300 degrees of rotation, allowing for a large focus scale for precise focus pulls.

Wrapping it Up

Looking at these new cinema-style zooms, you can see where each manufacturer has chosen to emphasize what they feel is most important. They are all parfocal, and share a circular iris opening that creates more natural, rounded highlights, as opposed to the octagonal or even triangular highlights used in lenses of the past. They all feature large focus barrels, with significant amounts of rotation, providing for precise focus adjustments. Each manufacturer plays to its strengths: Fujinon brings an ENG-style zoom control to a cine zoom lens, and adjustable back focus adjustment. Canon stays true to its long history of producing high-quality lenses using fluorite glass, and continues its design philosophy of sharing the same gear positions and lens dimensions. Zeiss brings full-frame coverage, five interchangeable lens mounts, and the same Zeiss look as its primes. These lenses provide professionals with a wide choice of look, feel, and function—choices that just a few years ago were unavailable. This lets you to choose one system to build and develop your own look with, or the option to choose a different look for every project.