Why do waves appear when you photograph a monitor?

A monitor has tiny points of light arranged in a grid, and a phone camera sensor has tiny light-receiving cells arranged in a grid too. If their spacing or angles are nearly the same but do not match exactly, the dots line up with cells in some places and fall between them in others.

Aligned areas photograph as bright and misaligned areas as dark, producing thick stripes that were not on the original screen. It is the same effect as the broad waves you see when you overlap two insect screens.

The light-sensitive cells in our eyes are not arranged in a regular grid like a camera sensor. They are therefore less likely to form a regular mismatch with the screen's dots, so the pattern does not appear. For the same reason, a checked shirt can look as though stripes are rippling across it on TV. People appearing on broadcasts are often told to avoid clothes with fine patterns.

It's easy to blame a low-resolution phone, but expensive cameras get it too. Adobe explains that moiré can appear even in very high-resolution photos when the pattern and angle line up.

That's why many digital cameras place a thin layer called an optical low-pass filter in front of the sensor, deliberately softening very fine patterns the sensor can't capture cleanly. Nikon notes that its D800E, which drops this filter, is slightly sharper but more likely to show moiré than the D800, which has one. It's a trade-off between sharpness and wavy patterns.

Tilting the phone slightly, or moving a little closer to or farther from the screen, changes how the two grids misalign and makes the pattern fainter. Zooming in a little with your fingers, or setting the focus slightly off the screen, also helps.

Try this when you want to keep a lecture slide or TV subtitles. Scanning magazine photos can produce the same pattern, as printed dots overlap the scanner's cells, so some scanner software includes a feature to remove moiré. If it's your own phone or computer screen, simply taking a screenshot gives the cleanest result.