Oscillating artifacts at image edges—sharp sharpening filters or poor demosaicing cause it. Kills fine detail.
When sharpening in post-production or directly in-camera, a visual phenomenon that frustrates every DoP quickly arises: bright and dark lines oscillate around image edges, especially where contrast occurs. This is ringing — and it systematically destroys detail. The cause lies in the mathematical nature of sharpening filters: they amplify high frequencies with such brutality that they create overshoot, which lays itself around the edges like a wave pattern.
In practice, this happens on several levels. Firstly, during demosaicing: when the raw engine (whether in Arri cameras, Sony, or Red) reads color information from the Bayer pattern, it has to interpolate. Aggressive demosaicing algorithms can already cause ringing here — especially with fine lines or textures. Secondly, during in-camera sharpening: many cameras offer sharpness controls. Turning these above 3-4 guarantees ringing in the highlights and shadows. Thirdly — and this is the most common source — during color grading or post-production in the edit with Unsharp Mask or similar filters. A radius of 0.5 pixels at 150% strength can already be enough to create visible halos.
The symptoms are unmistakable: on a sharp edge — such as dark text on a light background or a face with strong rim lighting — you see thin, bright streaks on the dark side and thin, dark streaks on the light side. On closer inspection, these even oscillate slightly. With moving material, this becomes particularly distracting because the artifacts then feel animated.
Prevention is the best remedy. Work with moderate sharpening values or turn off in-camera sharpening completely when recording raw — this is best practice anyway. In the edit: use Unsharp Mask with a very small radius (under 0.3) and lower strength (under 100%), or don't sharpen at all and instead rely on clean focusing techniques and good optics. A touch of local sharpening only on specific areas is more controlled than global sharpening over the entire image. Some graders work with high-pass filtering — this is more subtly controlled than Unsharp Mask.
On set: If a camera exhibits ringing, you keep it. It can no longer be undone in grading. So: perform tests with your system, document sharpening settings, and err on the side of caution in case of doubt.