Stair-stepping on fine lines and patterns from insufficient resolution — camera sensor or downsampling produces moiré and flicker. Optical filtering before sensor minimizes it.
Fine lines start to staircase, regular patterns begin to shimmer — this is spatial aliasing, and it happens sooner than you might think. Your camera's sensor cannot resolve every minute detail that the optics deliver to it. As soon as structures become smaller than two pixels apart, the sensor loses the information. The result: moiré effects, stair-step patterns on edges, a nervous shimmer on fine fabrics or architectural details. The problem isn't in the edit — it's already present in the raw footage.
The physics behind it are simple: the sensor samples the optical information at discrete pixel points. Anything that happens between these points is lost. If a frequency enters the image that is finer than the Nyquist limit of the sensor (roughly: half the pixel size), aliasing occurs — the high frequencies "fold back" and create visible artifacts. With modern high-resolution sensors (6K, 8K), the phenomenon is less dramatic, but in 4K productions with long focal lengths and a large depth of field, it occurs regularly. Particularly insidious: the actor's tight shirt, the house's roof tiles, the patterned wallpaper — anything can start to shimmer.
On set, aliasing is most effectively combatted before the sensor. Optical anti-aliasing filters (low-pass filters) placed before the sensor slightly diffuse fine structures — at the cost of minimal sharpness, but suppressing high-frequency artifacts. Many digital cameras already have these filters built-in (or offer different strengths). Those working without filters risk problems only during DCP mastering or when downsampling for TV delivery. Post-processing aliasing in the edit is tedious: you can work with subtle low-pass filters or slightly softened layers, but you lose genuine image detail in the process.
Practically speaking: test your camera with fine patterns — fine stripes in the background, geometric shapes — before shooting at high resolution with narrow focal lengths. The combination of sensor pixel size, optical filter design, and recording format determines how susceptible your setup is to aliasing. Some DoPs deliberately use slight diffusion or Promist filters — not just for look, but also to reduce susceptibility to aliasing. An overly aggressive filter costs sharpness; one that is too weak lets artifacts through. Finding the balance is part of the technical craft.