Many photographers who are entering the realm of motion capture have encountered an unexpected problem: the terminology. It’s like Vincent describing Europe to Jules in Pulp Fiction—everything is just a little bit different. In this brief article, we outline a few of the differences you may find confusing if you have limited or no motion-capture experience.
People talk about a "cinematic look" a lot these days, and the conversation frequently centers around the lenses. Cinema lenses have a number of key differences from still photography lenses. That’s because primes and zooms built specifically for video have different requirements than still lenses. Oversized, "declicked," freely rotating barrels are included for making subtle changes to focus and aperture while recording a take. Often, the casings are also big enough to add focusing marks to be able to rack focus between two subjects during a take. They also have geared focus for working with precise manual-focusing systems like follow-focus units. Still lenses, on the other hand, are built to be as efficiently compact as possible, so they’re lighter and the rotation is much more rapid.
Though comparable, cinema lenses also use T-stops rather than ƒ-stops to denote aperture. These T-stops and barrel diameters are often matched across a range of lenses to minimize any changes that must be made with lens peripherals like matte boxes, follow-focus units or rig systems while swapping focal lengths during filming. (See below for an explanation of T-stops.)
Even with high-definition video, it’s recommended that you stay with professional-level lenses, and that’s a 2-megapixel file compared to an 8-megapixel file with 4K. Resolution limits for a lens are shown through MTF charts, which use line-mm pairs to compare two distinct lines and the point at which they converge in a set of optics. At this point, they can no longer be discerned as separate points of detail, the resolution limit of the lens.
Still cameras pack a lot of resolution into sensors, and lenses need to be capable of resolving to these levels. Canon’s family of EF- and PL-mount Cinema Prime lenses are designed for 4K resolution. Currently, the lineup includes compact zooms, primes and CINE-SERVO lenses, which have a detachable Servo Drive Unit for automatic zooming during news-gathering types of events or quiet manual zooming during filming. Their set of Cinema Primes currently ranges from the CN-E14mm T3.1 L F to the CN-E135mm T2.2 L F.
Tokina is entering the cinema lens line with a few zoom models, the AT-X 11-16mm T3.0, 16-28mm T3.0 and 50-135mm T3.0. Each is based on their respective still lenses of the same focal lengths, but they feature manual focusing and better ergonomics for cinematography, as well as metrics for making focus changes. They will be available in Canon EF and PL mounts.
For Canon, Micro Four Thirds, Nikon and Sony mounts, Rokinon has been producing a number of very affordable cinema lens alternatives to lenses produced by the major manufacturers. Several are available as bargain-priced collections of essential primes on Amazon, like the Rokinon Full Cine Lens Kit with 85mm, 35mm, 24mm, 14mm and 8mm lenses, or the Super Fast T1.5 Cine Lens Kit with 35mm, 24mm and 8mm focal lengths.
In still photography, we don’t use the term lux that often. It’s a measure of illumination. We tend to use EV (Exposure Value) or lumens more than we use lux, but it’s frequently used in filmmaking. It’s a measure of light over a given area. As a unit, lux is equal to one lumen per square meter.
With film-based motion cameras, the shutter is a rotating semicircular disc. As it spins, the film is exposed as it passes through the camera gate. This also determines the shutter speed. By adjusting the shape of the semicircle, the effective shutter speed changes. That’s the shutter angle.
With digital cameras, the shutter works differently, but you’ll still hear motion people discussing shutter angles. It’s the key adjustment for controlling blur in the footage. In motion capture, some blur is frequently desired because it gives the footage a smooth and more pleasing look.
You’re intimately familiar with an ƒ-stop, and a T-stop is pretty much the same thing. T-stops are more common in motion capture, and are particularly familiar to filmmakers who have used film cameras. T-stops are more accurate than ƒ-stops for determining exposure because T-stops are ƒ-stops that are adjusted to account for light-transmission efficiency. In an optical system, the components hinder the amount of light passing through by a slight amount. For motion capture, where consistency between lenses from shot to shot is critical, T-stops are preferred. That doesn’t mean you can only use lenses with T-stops, of course, but be aware that when you’re shooting motion and you change lenses, even if you set the same ƒ-stop, the exposure likely will shift slightly.
These are a few of the particular differences between still photography and motion terminology. There are plenty of other semantic obstacles between the two worlds, but at the end of the day, the motion and still worlds are more similar than you may think.