Why your indoor iPhone video flickers, and the frame rate that fixes it

Where the bands come from

Mains electricity alternates, and most indoor lighting brightens and dims along with it — many LED and fluorescent fittings pulse rather than glowing steadily. The pulsing is far too fast for your eyes, which average it into constant light.

A camera does not average. It samples the scene once per frame, and if the sampling rate does not line up with the pulsing rate, successive frames catch the light at different points in its cycle. Some frames land bright, some dark, and because the sensor reads the image progressively from top to bottom, the mismatch shows up as bands drifting through the picture rather than the whole frame pulsing.

The fix: match the frame rate to the grid

When your frame rate divides evenly into the mains frequency, every frame samples the same part of the light cycle, and the flicker vanishes. Which number you need depends purely on where you are:

• 50 fps — countries on a 50 Hz grid: Europe, most of Africa, Australia, and most of Asia.
• 60 fps — countries on a 60 Hz grid: the USA, Canada, Mexico, Japan, South Korea and Taiwan.

Set it on the viewfinder: tap the FPS value in the top row of controls and choose 50 or 60. It is worth setting once for where you live and leaving it there — this is the single most common cause of indoor footage that looks subtly wrong for no obvious reason.

Why 24 fps often makes it worse

Cinema shoots at 24 fps, so people reach for it expecting a filmic result. Under artificial light on either grid, 24 fps does not divide cleanly into the pulsing and is a reliable way to introduce flicker. Outdoors in daylight it is fine — daylight does not pulse — but indoors it fights the lights.

There is a second reason the guides steer away from it. The cinematic quality people associate with 24 fps comes mostly from lighting, lenses and composition, not the frame rate. A well-lit 50 fps clip reads as far more professional than a flickering 24 fps one.

When the flicker survives the fix

Occasionally a light still misbehaves after you have matched the frame rate. Cheap LED bulbs and dimmed fittings are the usual culprits: dimmers chop the waveform in ways that no longer track the grid frequency cleanly, and some inexpensive drivers pulse at their own rate entirely.

• Undim the light if you can, or swap the bulb — a dimmed LED is the most common stubborn case.
• Turn off one offending fixture and rely on the others, or move so the flickering source is out of frame.
• Shoot near a window and let daylight dominate, since daylight cannot flicker.

Screens in the shot behave the same way

A television, computer monitor or laptop in frame has its own refresh rate, and it beats against your frame rate exactly as pulsing lights do. The symptom looks slightly different — the screen itself darkens, brightens or shows a band scrolling through it while everything around it looks fine.

Matching the mains frequency fixes most cases, because monitors in a given region are usually built around the same figure. When it does not, angle the shot so the screen sits outside the frame, or accept it and cut around the affected moments. This is worth checking deliberately whenever you film someone working at a desk.

Check it on the right screen

Flicker is frequently invisible on the phone during recording and obvious on a television or monitor. The phone's own display refresh can mask the banding, which is how people end up with an entire shoot ruined by a problem they never saw at the time.

Before a long take under unfamiliar lighting, record ten seconds, then watch it back full-screen. If bands are drifting through, change the frame rate and shoot the test again — it costs almost nothing compared with discovering it later.

The frame rate setting and shutter behaviour are separate things. The recommended workflow leaves Shutter on Auto — check that the lock icon beside the shutter value is transparent rather than blue — and fixes flicker with frame rate alone.