A time-lapse is governed by three numbers, and fixing any two determines the third. Interval, total shooting duration and final clip length are locked together by simple arithmetic — the skill is choosing an interval that suits what is actually moving in the frame, then living with the shooting time that implies.
The arithmetic
Frames = shooting duration ÷ interval. Two hours is 7,200 seconds; at a 25-second interval that is 288 frames.
Clip length = frames ÷ playback frame rate. 288 frames at 24 fps is 12 seconds.
Speed-up factor = interval × frame rate. A 25-second interval played at 24 fps runs 600 times faster than real life.
Run that backwards when you are planning. If you want a ten-second clip at 24 fps you need 240 frames, and if the event you are shooting lasts forty minutes, your interval is 2,400 ÷ 240 = 10 seconds. Almost every time-lapse planning question is one of those three rearrangements.
Choosing the interval
The interval should be short enough that the subject does not jump between frames, and no shorter. Too short wastes card space, battery and shutter actuations; too long produces strobing, where a moving object appears in a different place each frame with nothing connecting them.
| Subject | Typical interval | Notes |
|---|---|---|
| Fast clouds, busy street | 1–2 s | Anything moving visibly second to second |
| Slow clouds, crowds | 2–5 s | The general-purpose range |
| Sun across the sky, shadows | 15–30 s | Long shoots; watch battery |
| Stars and star trails | 20–30 s | Interval is capped by the exposure itself |
| Construction, plants growing | 5–30 min | Days to weeks; needs mains power |
| Traffic at night | 1–4 s | Long exposures blend headlights into streaks |
The rule of thumb that actually works: watch the scene for thirty seconds and ask how long it takes for the main moving element to travel its own width. Divide that by ten or so, and you have your interval.
Shutter speed matters as much as interval
This is the part beginners skip. In normal video, a 180-degree shutter gives motion blur that reads as natural. A time-lapse shot at 1/500 second per frame gives you 288 razor-sharp stills played in sequence, and the result stutters horribly — each frame is a frozen instant with nothing bridging the gap to the next.
The fix is to drag the shutter so each frame carries some blur. The ideal is a shutter time close to the interval itself, which in daylight means heavy ND filtration. A common working target is a shutter of roughly half the interval — a 4-second exposure on an 8-second interval — which gives noticeably smoother motion. This is sometimes called the 180-degree rule for time-lapse, and it is the single biggest quality difference between an amateur and a professional sequence.
Flicker, and where it comes from
Flicker is the plague of time-lapse, and it has three common causes:
- Aperture flicker. An electronic lens re-stops the aperture for every frame, and the blade position varies by a tiny amount each time. Over 300 frames that variation reads as pulsing. The fix is to lock the aperture — either use a fully manual lens, or use the lens-twist trick where you set the aperture then partly disengage the lens so the body cannot drive it.
- Auto-anything. Auto exposure, auto white balance and auto ISO all make independent decisions per frame. Everything goes manual.
- Real-world flicker. Mains-powered lights pulsing at 50 or 60 Hz will beat against your shutter speed. Choose a shutter that is a whole multiple of the mains cycle.
Residual flicker can be removed in post with a deflicker pass, but it is far easier to prevent than to fix, and heavy deflickering softens the image.
Budgeting cards and batteries
Frame count times file size gives you the card requirement, and it adds up faster than people expect: 288 raw frames from a 45MP body is around 25GB. A full day-length sequence at a 10-second interval is 8,640 frames.
Battery is usually the binding constraint on long shoots. Live view and image review are the two biggest drains, so turn both off; the camera should wake, expose, write and sleep. For anything past a couple of hours, use a dummy battery and a power bank rather than hoping.
Day to night: the holy grail
A sequence that runs from daylight into darkness spans something like fifteen stops, which no fixed exposure can cover. The two approaches are bulb ramping, where an intervalometer walks the exposure up gradually during the shoot, and shooting raw with auto ISO within a controlled range and correcting the ramp afterwards.
Both are fiddly. If you are attempting one, note that the interval has to grow as the exposure grows — you cannot shoot a 25-second exposure on a 20-second interval — so the speed-up factor changes across the sequence and needs smoothing in post.
Adding movement
A motion-control slider or pan head turns a static time-lapse into a moving one, and changes the interval calculation only in that the rig has to complete its move inside the same shooting window. Most controllers work in shoot-move-shoot mode: expose, advance, settle, expose. The settle time has to fit inside the interval, so slider work generally pushes you toward longer intervals than you would otherwise choose.
Run the numbers
Plan interval, frame count and final clip length together, with a suggested shutter time for natural motion blur.
Related reading
The motion-blur reasoning comes from Shutter Speed and Motion Blur and Frame Rates. Dragging the shutter in daylight needs the filtration covered in Fixed vs Variable ND Filters. For night sequences see Astrophotography Exposure, and for card and drive planning, the Data Calculator.