Why does dual camera video flicker under indoor lighting?
Short answer: many indoor lights are not actually a steady source of light. Fluorescent tubes and a lot of LED bulbs run on alternating current, and instead of glowing at a constant brightness they pulse on and off dozens or a hundred-plus times a second — fast enough that your eyes average it into something that looks steady, but not fast enough to fool a camera sensor reading the scene one exposure at a time. When a video frame happens to be exposed during a dimmer part of that pulse, the frame comes out darker than its neighbours, and strung together that shows up as flicker or a slow band moving through the picture.
This is a property of the light, not a fault in a phone or an app, and it happens to any camera recording at a fixed frame rate. Dual-camera recording adds one more wrinkle worth understanding, because the front and back cameras are not one system reading the same clock — they are two, and that has a specific effect on how the flicker actually shows up in the file.
Where the pulse in "steady" indoor light comes from
Household electricity is not a flat, constant voltage — it swings back and forth many times a second, and a light bulb wired straight to it swings its brightness right along with it. An old-style fluorescent tube is the most obvious case, but plenty of cheap LED fixtures do the same thing, because a simple, inexpensive driver circuit lets the bulb’s brightness track the power line instead of smoothing it out. Daylight and a battery-powered light do not have this problem, because neither one is riding an alternating current — they are as close to genuinely steady as light gets.
None of this is visible to a person standing in the room. The pulse repeats roughly a hundred times a second or more, well past the point where human vision can follow the flicker and see anything but a constant glow. A camera has no such limit — every frame it records is a single, short exposure taken at a specific instant, and an instant can land anywhere on that repeating brightness curve.
Why a fixed frame rate is either lucky or unlucky
A video camera does not record continuously; it takes a fixed number of separate exposures every second and calls that a frame rate. Whether those exposures happen to line up cleanly with a light’s pulse is mostly a matter of arithmetic. If a whole number of light pulses fits neatly inside the time each frame is exposed for, every frame ends up catching the same average brightness and the flicker disappears into the footage. If the pulses do not divide evenly into the frame rate, each frame catches a slightly different slice of the cycle, and the file ends up with a visible pulse or a soft band drifting through the image, because consecutive frames are not actually equally bright.
This is why the same fixture can look perfectly steady on one camera and flicker on another — it comes down to which frame rate each one happens to be locked to, not how good the camera is. DualCam locks both cameras to a fixed 30 frames per second for the whole recording, with no option to change it, so whether a given room’s lighting lines up with that rate is entirely down to the room, not a setting you can reach for.
Two cameras, two chances to catch it differently
A dual-camera recording does not run one shared exposure for the whole frame. The front and back cameras are separate hardware, each metering the scene it can see and choosing its own exposure independently — the same reason the two halves of a dual-camera shot can end up a different brightness or a different colour, covered in their own guides. Frame rate is locked the same for both, but the exact instant each camera’s sensor reads out is not synchronised between them, so under a flickering light the two cameras can catch a different point in the same pulse.
The practical result is that flicker in a dual-camera recording is not always symmetric. One half of the frame can show a visible pulse while the other looks steady, or the two can pulse slightly out of step with each other, because nothing coordinates the two capture pipelines beyond the moment their images are drawn into one composited frame. Point both cameras at the exact same light source and they can still disagree.
Not the same problem as filming a screen
A moving band across a TV, monitor or laptop in frame is a related but different phenomenon, covered in a guide of its own: that one comes from a screen redrawing itself on its own cycle while the camera reads its sensor on a different one. Indoor lighting flicker does not need a screen anywhere in the shot — it comes from the light itself, so it can show up in a room with no electronics in frame at all, and it lands on the subject and background generally rather than as a band confined to one rectangle in the picture.
What actually helps
- Prefer daylight or a battery-powered light where you can. Neither one is tied to the power line, so neither one has a pulse for a fixed frame rate to catch out of step.
- If you are stuck with a fluorescent tube or an unfamiliar LED fixture, check the live preview before you commit to a take — the frame you see is the frame that gets recorded, so a pulse visible on screen for a few seconds is a pulse that will be in the file.
- If one half of the frame is flickering and the other is not, try repointing just that camera at a different light in the room rather than assuming the whole shot is affected — the two cameras are catching the light independently, so fixing one side does not require moving the other.
- Do not expect a colour filter to fix it. DualCam’s filters are applied to the finished, composited frame, the same look on every frame — they can shift a colour cast, but a flicker is a difference between frames, not within one, and restyling a single frame does not touch how bright the frame next to it is.
Common follow-up questions
Is flicker under indoor lighting a sign something is wrong with my iPhone or the app?
No. It comes from the light itself pulsing faster than the eye can see, and it happens to any camera recording at a fixed frame rate under that kind of fixture — a phone, a dedicated camera, dual-camera or single.
Can I remove flicker afterwards in editing?
Not reliably with an ordinary video editor. Removing flicker properly means analysing and correcting the brightness of each frame individually, which is a specialised, frame-by-frame process that a colour filter or a simple brightness adjustment does not do. It is much easier to avoid at the point of recording than to fix afterwards.
Why does only one side of my picture-in-picture flicker and not the other?
Because the front and back cameras expose the scene independently rather than sharing one reading. Under a flickering light, each camera can happen to catch a different point in the same pulse, so one half can show a visible flicker while the other looks steady.
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