Frame rates and cadence: why old film looks fast
Before anything else, a decade has a rhythm: how many pictures a second the camera took, and how long each one was exposed.
A camera turned by hand
The first movie cameras had no motor. The operator turned a crank, and every turn pulled a few frames past the lens. There was no fixed standard: operators aimed for something between about 14 and 18 frames a second, and a steady hand was part of the job. Slow down on a dull scene, speed up when the horses came past, and the film kept the record of it.
Turning slowly also saved film, which was expensive. Sixteen frames a second was enough to fool the eye into seeing motion, and it is the rate most of the silent-decade technologies on the dial use.
Why old film looks fast and jerky
Two different things are going on, and they are easy to confuse.
- It looks fast because of how it is shown today. When sound film arrived at the end of the 1920s, projectors settled on 24 frames a second, and later television and video transfers ran at their own fixed rates. Film shot at 16 and played at 24 runs half as fast again: people scurry, and a walk becomes a trot. That speed-up was never in the original footage.
- It looks jerky because there are fewer pictures per second. At 16 frames each picture stays on screen longer, and a moving hand jumps a little further from one to the next. That stutter is in the footage itself, whatever speed you play it at.
The Time Travel Camera reproduces the second effect and not the first. Your clip keeps its real duration, so people move at their own speed, but the motion has the step of the decade.
How the app holds frames inside a 24 fps file
Every video the app records is a 24 frames-a-second file, whatever decade you choose. That is the rate your iPhone captures at, and the rate the file plays at everywhere you share it.
A slower cadence is made by holding frames. For a 16 fps technology, the app works out which of the 24 frames in each second would have been a new picture on a 16 fps camera, renders those, and repeats the last picture in between. In practice two out of every three frames are new and one is a repeat, so you see 16 different pictures a second, spread evenly. The same rule gives 18 fps for Super 8 and 14 or 15 fps for the roughest hand-cranked looks.
A few technologies use a fixed step instead. Single-Frame Animation in the 1900s holds each picture for three frames, which gives 8 pictures a second, the jerky rhythm of early animated film. Surveillance Camera Tape in the 1990s holds each picture for six frames: four pictures a second, close to the 5 fps the recipe was written for, as a time-lapse recorder that had to fit a day on one tape.
Faster rates are not reproduced
Some recipes were written for rates above 24: the motorised studio camera of the 1890s at 40, video at 29.97 or 30, modern phones at 60, and one 2030 technology at 120. The app can't add frames that the camera never captured, so all of these play at 24 frames a second. What you get from them is everything else about the look: the picture, the tape, the sensor, the color.
That is also why some decade pages list "24 fps" for a technology that historically ran faster. The page tells you what your file will contain.
The shutter
A movie camera's shutter is a spinning disc with a gap in it. While the gap passes the gate, the film is exposed; while the solid half passes, the claw pulls the next frame down. With the common half-open disc, each frame is exposed for half the time between frames. At 16 frames a second that is about a thirty-second of a second, long enough for a waving hand to blur.
The app doesn't model the shutter disc in its video technologies: the blur in a CINE clip is the blur your iPhone caught. Exposure time shows up in the photo styles instead. Most of them carry a motion blur strength, a short horizontal smear, as if the subject or the camera moved during the exposure. It is strongest in the early decades, when plates and roll film were slow and a portrait sitter had to hold still, and it fades to almost nothing by the 1980s.
Interlacing: the video picture in two halves
Television and analogue camcorders didn't record whole frames. They recorded fields: first the odd lines of the picture, then, a sixtieth or fiftieth of a second later, the even lines. Two fields made one frame. On a television that was invisible. Paused, or on a modern screen, anything moving shows a comb of alternate lines, each half caught at a slightly different moment.
Many video technologies from the 1980s to the 2000s are interlaced in their recipes, and three of them show it: Black-and-White Television in the 1950s, Surveillance Camera Tape in the 1990s and Interlaced Consumer Video in the 2000s. In those, alternate lines of each frame show the frame captured just before, so moving edges get the comb and still areas stay clean. It is a picture of interlacing inside a progressive 24 fps file: the file itself is not interlaced, and the field rate is not real.
Frames that never arrived
One more rhythm belongs to the internet decades. A video stream that can't keep up skips frames and shows the last one again. One technology, Mobile Live Stream in the 2010s, carries this as a strength called frame drops: now and then a frame repeats where a new one should be. Like the other defects, it follows DEGREE.