Moire

You know the problem: The actor is wearing a shirt with a fine checkered or pinstripe pattern, and suddenly shimmering waves dance across the fabric on the monitor — that's moiré. It arises from the interference between your camera's sensor resolution and the fine structure of the subject. The two grid frequencies are close to each other, partially cancel each other out or reinforce each other, and the eye perceives a psychedelic pattern that has no business being in the material.

The technical side: Modern sensors have an optical low-pass filter (AA filter) designed to mitigate this effect — but at very high resolutions and extreme close-ups, it will still hit you. Some cameras — RED is known for this — deliver sensors with no or a weaker AA filter for maximum image sharpness; the result: moiré becomes more frequent and prominent. You can't simply fight it in post-production without massive quality loss.
Practical solutions on set: Light diffusion (1/4 or 1/8 diffusion in front of the lens) is your first choice — it takes enough edge sharpness from the subject without softening it. Sometimes a tiny focus pull forward or backward can help shift the critical frequency. In a serious situation — and this is important — talk to costume and direction: Can the tight fabric be replaced with something with a broader structure? A dark blue sweater instead of the pinstripe shirt costs you zero image quality. If ND filters come into play: Moiré often intensifies in very bright conditions because the sensor is working hard. A good ND can actually help here because without it, you would be overexposed and would have to push the saturation.

Monitoring trick: Use an external Full HD monitor to detect moiré susceptibility early — not every camera shows it the same way. RED and ARRI sensors behave differently. Also, consider side-mounted displays: Interference patterns often reveal themselves there earlier than on your HMI.