3.5 Video⧉
Video is a stack of exposures, so most of it forward-references the dedicated Motion/Video chapter; here we name just the controls a stills photographer meets when they hit the record button.
- Frame rate and shutter angle. Video sets the exposure time as a fraction of the frame interval. The 180° shutter-angle convention, with the shutter open for about half the frame time, gives the "natural" amount of motion blur that reads as cinematic; too short a shutter looks stuttery, too long looks smeared. A high frame rate (shot then played back slower) gives slow motion.
- Rolling shutter and "jello." A complementary metal-oxide-semiconductor (CMOS) sensor reads out row by row (the sensor chapter), so during fast motion, such as a quick pan or a spinning propeller, the top and bottom of the frame are captured at slightly different instants and the image skews or wobbles, the "jello" effect. A global shutter (every pixel exposed at once) avoids it but is rarer and costlier.
- Log profiles and grading. Shooting a log (flat, low-contrast) profile preserves dynamic range across the frame, to be color-graded in post. It is the video cousin of shooting raw and applying a tone curve later.
- Codecs and bitrate. Intra- versus inter-frame compression, 8- versus 10-bit, chroma subsampling (4:2:0 vs 4:2:2), and bitrate all trade file size against editing and grading headroom.
3.5.1 Cinema lenses⧉
A cinema lens does the same optical job as a stills lens but is built to a different specification, because a motion picture is shot, cut, and graded as a sequence and every shot has to match its neighbors. The differences are almost all about consistency and control rather than raw sharpness.
- Marked in T-stops. As in the exposure section, each lens is calibrated for transmission and engraved in T-stops, so a set exposure holds when you swap lenses. The aperture (iris) ring is de-clicked and continuous, letting a focus puller ramp the stop smoothly mid-shot instead of jumping between detents.
- No focus breathing. An ordinary lens changes its field of view slightly as you refocus, so a focus pull from foreground to background looks like a small, distracting zoom. Cinema lenses are optically corrected so the framing stays put through a pull. That "breathing" is now such a recognizable tell that some mirrorless cameras add an electronic breathing compensation to fake the cinema behavior.
- Parfocal zooms. A cinema zoom holds focus across its entire range, so an operator can zoom during a take without the image going soft. Many stills zooms are merely varifocal and drift out of focus as they zoom.
- Long, smooth, repeatable controls. Focus rotates through a long throw (about 270–300°, versus a stills lens's short arc or focus-by-wire), with a firm mechanical feel and hard stops, so a focus puller can set precise witness marks and hit them take after take. Standard 0.8-module gear rings at standard positions let follow-focus units and lens motors clamp onto any lens in the set.
- Matched across a set. A set of cinema primes is tuned to render color, contrast, and out-of-focus rendering identically, and to share a common front diameter and length, so a matte box and rig transfer from one focal length to the next and intercut shots look like one camera. Stills primes bought over years rarely match this closely.
- Built for the rig, not the pocket. Manual everything, heavy metal barrels, standardized mounts (PL, LPL) with consistent flange geometry, and usually no autofocus or stabilization. The result costs several times a comparable stills lens, and earns it on a set where a single missed focus pull ruins a take.
The optics inside are often much the same designs; what you pay for is calibration, mechanical precision, and the discipline of a matched, repeatable set. It is the lens half of the same "shoot it flat and controlled, sort it out in the grade" philosophy behind log profiles above.
The algorithms behind all of this, and the full treatment of motion, live in the Motion/Video chapter; the control list above is deliberately brief.