Film grain overlay

01 · Grain

What film grain is

Film grain is the picture itself. Each speck is a developed silver crystal (black and white film) or the cloud of dye that formed round one (color film). Crystals come in a spread of sizes, sit at random and clump, so grain has no grid and no single size. Digital noise is something else: a measuring error laid over the picture, one value per pixel on the sensor’s grid.

Noise with a new value at every pixel and no clumps is the signature of a sensor, as the AV1 film grain paper puts it, and Newson’s grain model calls grain without clumping “clearly unrealistic”. A plain Add Noise filter makes exactly that. Two other giveaways of drawn grain are a grid (square cells show as a faint screen and streaks) and one strength at every tone.

02 · Editing

How to use a film grain overlay

The same steps in any editor with layers or tracks:

1. Make it the size of your image. The same pixels as the photo or the video frame. Scaling an overlay changes its grain: smaller makes it finer and weaker, larger blurs it.
2. Put it above the picture. A layer over the photo, or a clip on the track above the video, covering the whole frame.
3. Set the blend mode to Overlay. In Overlay and Soft Light, mid gray (128 of 255) changes nothing: grain lighter than gray lightens the picture under it, darker grain darkens it. Soft Light is about half as strong.
4. Set the amount with opacity. Start strong and bring the opacity down. Judge it at 100 %, and again at the size the picture will be seen.
5. Keep the grade above the grain. On film the grain is in the negative, and the print’s curve and color shape it. If your editor allows, put the grain layer under your color and contrast adjustments, not over them.

The blend mode is in the layers panel in Photoshop, Affinity Photo and GIMP; under Opacity in Premiere Pro’s Effect Controls; in the Inspector’s Composite Mode in DaVinci Resolve; under Blend in CapCut. An editor without blend modes cannot use a gray overlay: use Your photo in the tool above instead, which puts the grain into the picture itself.

03 · Tone

Why an overlay is only an approximation

On a color negative the grain is in the negative’s density, which varies by a nearly constant amount at every exposure, which in the final picture is a constant error in stops of exposure. A stop takes more of the sRGB range in the light tones than in the dark ones, so the grain grows through the mid tones, and the print’s curve squeezes it out into white. FilmConvert’s developer sees “more visible grain at 80 % white than at pure white”.

A blend mode knows only the brightness under it. Overlay puts the grain in at full strength at mid gray and less in a straight line towards black and white; Soft Light puts in about half, and less again in the light tones. Grain strength by tone, as a percentage of each one’s full strength:

Our arithmetic. Film: Clack’s grain curve, 100 % at its peak, a light mid tone. Overlay and Soft Light: the share of the layer’s grain each passes to the picture, by the standard blend formulas, 100 % being Overlay at mid gray. Editors’ Soft Light formulas differ a little.

So an overlay takes the grain out of the light tones, where film has the most, and a layer strong enough to show in a bright sky is too strong in the mid tones. Your photo, in the tool above, follows the film’s curve instead. It also makes the shadows coarser: underexposed parts of a color negative were recorded by its fast emulsion alone, with its large grains (Shutterangle; Dominic Case). Strength peaks in the light mid tones, coarseness in the shadows.

On video, an overlay is frozen

Every frame of a film is a new piece of film, so real grain is new in every frame. A still PNG over a clip stays put while the picture moves through it, one of the giveaways of drawn grain. For moving grain, use a grain clip or a grain effect that draws a new pattern each frame.

04 · Size

Grain size: 35 mm at 12 MP

Measured color negative grain has two scales: dye clouds about 6.5 µm across, and clusters of them about 15 µm across (Stephenson and Saunders, 2007). A 35 mm frame is 36 mm wide. Drawn 4080 pixels wide, a 12 MP photo, one pixel is 8.8 µm of film, and the grain is a gaussian about 0.25 and 0.55 px in radius: a texture at the scale of a pixel, not specks you can count.

Our arithmetic from Stephenson and Saunders’ measured scales, on a 36 mm frame.

Below half a pixel the tool, as Clack, draws the grain at half a pixel and weakens it in proportion: what finer grain averages to at that size. On HD video, 35 mm grain is a quarter of a pixel or less and mostly averages away. Seen whole on a phone, a 12 MP photo’s grain becomes a fine texture too; what still reads is its strength against tone, its color, and the absence of any grid or streaks.

Grain of a pixel and more is coarser than 35 mm at that size: a grainier film, a push, a crop. Kodak puts Tri-X 400 at a granularity of 17 and notes that pushing raises graininess. Choose it for the look; it is not what 35 mm film looks like at that size.

05 · Clack

How Clack does grain

Clack is a film camera app for Android and iPhone. Grain is part of each film, not a layer on top: the finder shows it live and the saved photo has it. Your photo, in the tool above, is the same grain code without the film’s curve, so it grains a finished photo where Clack grains the film.

Before the film’s curve

Grain goes in before the film’s curve and color, so the curve shapes it as a print does.

Sized like 35 mm

Clumps of half a pixel in radius on the 12 MP photo, at two scales, round, with no grid. Full screen it is a fine texture, not specks.

Strength by tone

Constant in stops: strongest in the light mid tones, gone into white.

Coarser in the shadows

Dark parts get larger clumps, as an underexposed negative does.

Faint color

Each dye layer’s own clumps, on the coarse scale only, blue the most. No pixel-sized color speckle.

Follows the light

The higher the ISO the camera needed, the stronger the grain, as film shot in a dim room and pushed: ×1.15 a stop above ISO 100, at most ×1.74 from ISO 1600.

New every frame

In the finder and in video the grain is drawn new for every frame, as each frame of film is new.

The film base, where the lab slip lives, runs on Android phones whose camera accepts a tone curve of its own. There each film also has its own grain (Gold 200 grainier than Portra 400, by Kodak’s Print Grain Index; Ektar 100 the finest), and rating a negative under on the slip strengthens it as the ISO does. On other phones Clack’s films run on the phone’s own frame, with the recipe’s grain.

Android · iPhoneComing soon to Google Play and the App Store. More about Clack