How to reduce noise in a grainy photo

Noise is the speckle that appears in photos taken in low light, and it gets worse the moment you brighten the shadows. Noise reduction smooths it away, and too much noise reduction smooths everything else away with it. The trick is finding the value where the grain goes and the detail stays.

Where noise comes from

A camera sensor counts photons. In bright light it counts millions per pixel and the random variation between neighbouring pixels is invisible. In dim light it counts far fewer, the variation becomes a large share of the signal, and the camera amplifies it all to make the photo look exposed. That amplified randomness is the grain. It is worst in shadows, in flat areas such as walls and skies, and in the blue channel, which sensors are least sensitive to.

How the slider works

A naive blur would smooth noise and edges alike. The Noise reduction slider in Raspy is edge-aware: for each pixel it averages the neighbours that are similar in brightness and ignores the ones that are not. A flat wall gets smoothed because all its pixels are alike; the edge of a face does not, because the pixels on either side differ. Higher values widen what counts as "similar", so finer detail starts to be treated as noise and smoothed away. That is the plastic look.

Steps

1. Do the exposure work first. Lifting Shadows or Exposure reveals noise, so there is no point judging it before.
2. Zoom in on a flat, dark area. Press + on a keyboard, or pinch on a phone. Noise is judged at 100%, not on the whole frame.
3. Raise Noise reduction until the speckle in the flat area calms down. Usually between 20 and 50.
4. Move to an area with fine texture: hair, fabric, foliage. If it has gone smooth, reduce the slider until the texture returns, even if some grain returns with it. Grain is a photographic look; smeared texture is a flaw.
5. Zoom back out. A little visible grain at 100% is invisible at the size the photo will be seen.

Sharpness and definition fight noise reduction

Sharpness crisps fine edges and Definition adds contrast to mid-sized detail. Both amplify whatever small variation they find, and noise is small variation, so raising either after Noise reduction brings the grain back. If a photo needs both, keep Sharpness low, use Definition instead of Sharpness where possible since it works at a larger scale, and revisit Noise reduction after. Raspy applies these three on luminance only, so they do not add coloured speckle, and the export scales them so the full-size file matches the preview.

Accepting some noise

A night photo that is a little grainy looks like a night photo. One that has been scrubbed clean looks like a painting. Most of the time the right amount of noise reduction is less than the amount that removes all the noise, and a good compromise is to reduce the noise in the shadows and leave the midtones alone, which is roughly what happens at moderate values anyway.