Where each one comes from
Digital noise is produced by the camera sensor and its electronics. It comes from the random nature of light arriving at each pixel and from the circuitry that reads the signal out. It is strongest when there is little light, which is why it shows up when you raise the ISO in a dark scene.
Film grain is produced by the film itself. Black and white film contains tiny silver halide crystals, and color film forms small clouds of dye where those crystals were. The crystals are random in size, shape and position. Faster films use larger crystals to capture more light, which is why higher-ISO film looks grainier.
How they look different
| Digital noise | Film grain | |
|---|---|---|
| Structure | Tied to the pixel grid, roughly pixel-sized and uniform | Irregular, clumped particles that are not aligned to any grid |
| Color | Often blotchy red, green and blue speckle (chroma noise) | Subtle chroma speckle that varies per color layer |
| Tonal response | Strongest in the shadows | Strongest in shadows and midtones, fading toward pure black and white |
| Scale | Fixed to the sensor pixels, so it shrinks when you downsize | Fixed relative to the frame, so it looks the same size at any resolution |
Why a noise overlay does not look like film
A typical grain filter lays one repeating noise texture over the image at a set opacity. It looks identical in the shadows and highlights, it is often the same pattern every time, and it looks different depending on how many megapixels the photo has. Those are exactly the things that make it read as digital noise rather than film.
A better simulation generates particles of varying size and clustering, gives each color channel its own pattern, weights the effect by tone, and scales the grain with the image. That is the approach FilmGrainLab takes.
Should you remove noise before adding grain?
Often, yes. If a photo already has visible sensor noise, adding grain on top stacks two textures and the result looks messy. A light denoise first, then grain as the final step, gives a cleaner and more film-like result. See how to add film grain to a photo for the full workflow.