Where noise comes from
Adobe's Denoise team explains that light itself is noisy — the number of photons reaching each pixel varies — and the sensor's readout adds more, mostly in the shadows [1]. Adobe's Photoshop guide lists the usual triggers: high ISO, underexposure and long shutter speeds in dark areas [2]. Canon adds that raising ISO amplifies the sensor signal, generating noise in the process [3].
Two kinds of noise
Luminance noise is grayscale grain that makes an image look grainy or patchy; colour noise shows up as coloured artefacts [2]. That's why every tool — Lightroom, Photoshop, GIMP — has separate luminance and colour controls, and why colour noise is usually removed more aggressively.
Why AI denoising keeps more detail
Classic filters smooth pixels, so they trade noise for detail — GIMP's Noise Reduction has a single strength slider for exactly that balance [4]. Adobe's AI Denoise, launched in April 2023, was trained to separate noise from detail on raw sensor data [1]; general AI restorers like Phota Enhance do the same kind of job on finished JPEGs by rebuilding texture rather than just blurring it.
