Frame Rates, Mains Frequency and the Regions They Belong To
Every delivery specification traces back to two things: the 24 frames a second film settled on, and the frequency of the electricity supply where the pictures were meant to be watched. A rate in frames per second and a system rate in hertz are the same count of events in one second, so a schedule may quote one and a broadcaster's spec sheet the other. The trouble starts where the lineages meet: 24 does not divide neatly into 50, colour television shaved a thousandth off the American rates, and lamps flicker at twice the mains frequency whatever the camera does.
Three families of numbers
24 came from the film gate
25 and 30 came from the wall socket
Colour bought the odd thousandth
Setting a Project Rate and a Shutter Before the Shoot
Enter the rate the delivery spec asks for
Type it exactly as written, fractional rates included — 23.976, 29.97 and 59.94 all go in as they are, and a comma is accepted where a spec sheet uses one as the decimal mark.
Read it as the picture rate in hertz
The figure carries over unchanged, and that is the number to set against the mains where you are shooting: 50 Hz lighting flickers at 100 Hz, 60 Hz lighting at 120 Hz.
Reverse it when the paperwork does
A system rate in hertz becomes an acquisition rate by typing into the other field or pressing the swap arrows — both sides are live, so a spec quoted either way round takes one entry.
Copy the value into the camera report
The copy button lifts the digits alone, without a unit, which is what a report field, a conform note or a timeline preset expects.
Delivery Frame Rates and Their Exact Fractional Values
The first column holds the labels; the second is what the equipment is really running. Anything derived from 1 000/1 001 is a repeating decimal, so treating 23.976 as though it were 23.976 exactly will eventually cost you a frame.
| Labelled rate | Exact value | Frame duration | System it belongs to | Pulldown to fields |
|---|---|---|---|---|
| 23.976 fps | 24 000 / 1 001 = 23.976 023 976… | 41.708 3 ms | 60 Hz territories | 3:2 → 59.94 fields/s |
| 24 fps | 24 exactly | 41.666 7 ms | Cinema origination | 3:2 → 60 fields/s |
| 25 fps | 25 exactly | 40.000 ms | 50 Hz territories | 2:2 → 50 fields/s |
| 29.97 fps | 30 000 / 1 001 = 29.970 029 970… | 33.366 7 ms | 60 Hz broadcast | 2:2 → 59.94 fields/s |
| 30 fps | 30 exactly | 33.333 3 ms | Online and 60 Hz capture | 2:2 → 60 fields/s |
| 50 fps | 50 exactly | 20.000 ms | 50 Hz high frame rate | Progressive, no pulldown |
| 59.94 fps | 60 000 / 1 001 = 59.940 059 940… | 16.683 3 ms | 60 Hz high frame rate | Progressive, no pulldown |
That thousandth is also why timecode has two flavours. A clock counting 30 labels a second against footage running at 29.97 gains 3.6 seconds every hour, so drop-frame timecode skips two frame numbers at the start of each minute except every tenth — 54 minutes × 2 = 108 numbers an hour — to keep the display honest. No frames are lost; only the labels are.
Fractional broadcast rates entered in full
A rate written out as 23.976 023 98 keeps its digits, because the read-out carries eight decimal places instead of rounding a 1 000/1 001 figure back to something tidy.
Picture rate beside the supply frequency
Expressing the acquisition rate in hertz makes the comparison with 50 or 60 Hz mains, and the 100 or 120 Hz flicker it produces, a single glance.
Values that paste into a conform note
Copying returns bare digits with no unit and no thousands spacing, which a timeline preset, camera report field or delivery checklist accepts without editing.
Frame Rate Questions From the Edit Suite
Where do 23.976 and 29.97 come from, and what does 1 000/1 001 mean?
When colour was added to a monochrome system already running 30 frames a second, the colour subcarrier had to sit where it would not beat against the sound carrier. The fix was to slow everything by 1 000/1 001, putting the line rate at 4.5 MHz ÷ 286 = 15 734.27 Hz, the field rate at 59.940 06 Hz and the frame rate at 30 000/1 001 = 29.970 03. Cinema material for the same territory inherited it, giving 24 000/1 001 = 23.976 02. Both are exact fractions, not the rounded decimals printed on menus.
How does 3:2 pulldown fit 24 fps into a 60 Hz system?
Sixty fields do not divide evenly by 24, but four frames do spread over ten fields: three from the first, two from the second, three from the third, two from the fourth. That is the 3:2 cadence. A 50 Hz system needs nothing so clever, since 25 frames map straight onto 50 fields as a plain 2:2. The cost of 3:2 is judder — alternate frames are held for different lengths of time, the faintly lurching look of film on older television.
Why does my footage band under studio or street lighting?
Discharge and LED lamps brighten and dim at twice the supply frequency — 100 Hz on a 50 Hz supply, 120 Hz on a 60 Hz one. If an exposure does not span a whole number of those pulses, each frame catches a different slice of the cycle and the picture pumps; with a rolling shutter the mismatch becomes spatial and bands crawl up the frame. Dimmed LED fixtures are worse, adding a much faster pulse-width flicker unrelated to the mains.
What shutter speed does a given shutter angle actually give me?
Exposure time is the angle divided by 360, divided again by the frame rate. The familiar 180° gives 1/48 s at 24 fps, 1/50 s at 25 and 1/60 s at 30. To stay clean under lighting, pick the angle landing on a whole flicker cycle: 172.8° at 24 fps is exactly 1/50 s, two pulses of a 100 Hz lamp, and 216° does the same at 30 fps. Under a 60 Hz supply the safe target is 1/60 s, or 150° at 25 fps.
How do I move a programme between 25 fps and 30 fps territories?
From 24 to 25 the traditional route is to run the whole thing 4.166 7 % fast, shortening the running time and lifting audio pitch by about 0.71 of a semitone unless corrected. There is no comparable trick between 25 and 30: the rates share no small ratio, so you either interpolate motion, which softens fast movement, or repeat and drop frames, which stutters. Originating at the target rate remains the only route with no compromise attached.
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