You point your phone at the TV, tap the shutter, and there it is: a dark horizontal stripe cutting across the image like someone dragged a thumb through wet paint. You shoot again. Same stripe, different position. Nothing is broken. Two timing systems are quietly disagreeing, and your photo is what that argument looks like.
Two clocks that refuse to agree
Every digital screen refreshes itself at a fixed rate. A typical monitor cycles through 60 complete redraws per second. A phone screen in Europe often runs at 50 Hz, a legacy of the local electrical grid. The exact number matters less than the concept: the screen is switching its pixels on and off faster than your eye can register, which is why it looks steady to you.
Your smartphone camera does not see it that way.
It uses a rolling shutter sensor, meaning it doesn't capture the entire frame in one simultaneous snapshot. It reads the image one horizontal row of pixels at a time, scanning top to bottom in sequence. On a typical modern phone, that full scan takes somewhere between 1/30th and 1/60th of a second. Fast. Not instantaneous.
So the top of your photo is captured at a slightly different moment than the bottom. If the screen completes a refresh cycle partway through that scan, part of the frame catches the screen mid-cycle, when certain rows of pixels are momentarily dimmer or fully off. The result is a band of darkness frozen in place by the camera's scan. It isn't blur, it isn't noise. It's a literal record of the screen's off-state.
The shutter speed trap
Whether banding appears, and how bad it gets, depends entirely on the relationship between your shutter speed and the screen's refresh rate.
If your shutter speed is an exact multiple of the screen's refresh cycle, the sensor finishes its scan just as the screen completes a full cycle. Clean image. Nudge the shutter speed slightly off that harmonic and the scan catches the screen mid-blink. One band. Nudge it further and you get two. This is why the banding seems to drift or multiply when you zoom or adjust exposure: you're accidentally shifting that timing relationship, like tuning slightly off a radio station.
Here's a concrete example. Two people photograph the same 60 Hz gaming monitor. One shoots at 1/60th of a second; the other shoots at 1/120th. The first gets a clean frame because the shutter duration exactly matches one full refresh cycle. The second, shooting twice as fast, captures only half a cycle and catches the screen mid-transition. Banding appears across the lower half of the shot. Same monitor, same room, completely different results.
The mismatch gets worse with moving content because the screen's luminance is changing row-by-row even within a single refresh cycle. A static image on a screen is forgiving. A fast pan in a video is not.
What people consistently misread
Most people blame the camera. A few blame the screen. Both are wrong, and honestly this is one of those cases where the industry has done a terrible job of explaining a problem it has no real incentive to solve. The actual culprit is the combination of the two devices, and specifically the assumption that modern hardware has quietly fixed this. It hasn't.
Rolling shutter sensors are cheaper, faster to read out, and nearly universal in smartphone cameras. Global shutter sensors, which capture every pixel simultaneously and eliminate the problem entirely, exist in professional cinema cameras and some specialized mirrorless bodies. They carry manufacturing costs and noise penalties that keep them out of phones, probably for another decade.
Another common mistake: assuming video mode suffers the same way. It usually doesn't, because video frame rates are typically locked to harmonics of common refresh rates. Phones shooting at 60 fps on a 60 Hz screen are already synchronized. Still photography, where you pick arbitrary shutter speeds, is where the problem bites.
So what do you actually do about it? Set your shutter speed to a value that divides evenly into the screen's refresh rate. Shooting a 60 Hz monitor: use 1/60th, 1/30th, or 1/15th of a second. Shooting a 50 Hz European television: use 1/50th or 1/25th. If your phone's pro or manual mode won't go that slow, bump the ISO to compensate for the dimmer exposure, or accept a slightly brighter image and pull it back in editing.
If the banding disappears completely, you've hit the harmonic. If a faint stripe remains, the screen itself may be running at a non-standard rate, which happens more than manufacturers will admit.
The stripe in your photo isn't a flaw in either device taken alone. It's what two perfectly functional systems look like when nobody bothered to tell them to listen to each other.