What is film grain

01 · Grain

What causes film grain

Film records light in crystals of silver halide. Development turns the crystals that caught light into specks of black silver; in color film each one also forms a cloud of dye, and the silver is then bleached away. What you see as grain is those specks and clouds and the clumps they form. Larger crystals catch light more easily, which is what makes a film fast, and why fast films are grainy.

Size

In measured color negative grain, about 6.5 µm (one dye cloud) and 15 µm (a cluster of them) (Stephenson and Saunders, 2007).

Pattern

At random, in a spread of sizes, clumped. In Newson’s grain model, clumping is “an essential visual feature” and grain without it is “clearly unrealistic”.

Color

Each of a color film’s three layers grains on its own. The blue-sensitive layer is the coarsest, about twice the green’s on Kodak’s Vision3 500T sheet (read off its chart), which shows as a faint yellow-blue mottle.

02 · 35 mm

How grainy is 35 mm film

Grain is measured on the film, but seen in the print or on the screen, so how much shows depends on the enlargement. Kodak’s Print Grain Index rates the grain of a print as people see it: 25 is the threshold of visibility, and 4 points is a just-noticeable difference. A 35 mm frame is 24 × 36 mm, so a 4 × 6 inch print already enlarges it about four times. The other makers publish RMS granularity, measured on the film itself; Kodak says the two cannot be compared.

Lower is finer in both measures. Grain in Clack: the Grain row on each film’s lab slip at box speed (section 06).

35 mm against medium format

From the same film, a 6 × 6 cm frame (56 mm on a side) needs 2.3 times less enlargement than a 35 mm frame (24 mm on its short side) for the same print: 1.2 doublings. Kodak’s own tables put about 20 points on the index on each doubling, so the medium format print is about 24 points less grainy (our arithmetic).

35 mm grain in pixels

A 35 mm frame drawn 4,080 pixels wide, a 12 MP photo, has 8.8 µm of film in each pixel. Dye clouds of 6.5 µm and clusters of 15 µm are then a texture at the scale of a pixel, not specks you can count (our arithmetic). Grain you can see speck by speck on a phone screen is a heavy crop, a much faster film or a push, or drawn grain that is too big.

03 · Exposure

What makes film grainier

* Our reading of Kodak’s tables (E-58, E-7022).

Where grain shows in a photo

Not evenly. On a color negative, grain varies the negative’s density by a nearly constant amount, which is a constant error in stops of exposure. A stop takes more of the picture’s brightness range in the light tones than in the dark ones, so grain grows through the mid tones, and the print’s curve squeezes it out into white and into black (our arithmetic). FilmConvert’s developer sees “more visible grain at 80 % white than at pure white”.

Coarseness is a different thing. The shadows of a negative were recorded only by its fastest layer, with the largest grains, so grain there is coarser, if not stronger (Shutterangle; Dominic Case on CML). “Grain is strongest in the highlights” and “grain is coarsest in the shadows” are both said, and both are right about different things. Slide film may run the other way: Case puts its larger grains in the highlights, which are its thinnest parts.

04 · Noise

Film grain vs digital noise

Sensor noise is a measuring error laid over the picture: one value per pixel on the sensor’s grid, colored, because each photosite errs on its own, and worst in the shadows, where little light fell. A phone then smooths it with noise reduction and sharpens what is left. On the phone Clack is tested on, a CMF Phone 2 Pro, the phone’s own JPEGs come out smooth even at ISO 3391: watercolor, with halos along edges, and no texture of their own.

Grain is the picture: no grid, a spread of sizes, shaped by the film’s curve, nearly colorless, and it dithers smooth gradients so they do not band. The film grain overlay page has the two side by side in a table.

05 · Real and fake

Real film grain and fake

Real film grain comes only from film: shoot it and print or scan it. Everything else is drawn, in an editor (Lightroom’s and Photoshop’s Grain, Add Noise), with an overlay, with a film emulation plugin such as FilmConvert or Dehancer, or by a camera app. What tells drawn grain from film’s:

The grid: Lagae et al. (2010). Per-pixel noise as a sensor’s signature: the AV1 film grain paper. Clumps: Newson et al. (2017).

How to add realistic film grain to a photo

1. Start from the right size. At full size, 35 mm grain on a 12 MP photo is finer than a pixel. Lightroom’s Grain (under Effects) has Size and Roughness sliders; keep the size low and judge it at 100 %.
2. Make it clump. Grain with a new value at every pixel reads as sensor noise. In Lightroom, Roughness makes the grain less uniform; after Photoshop’s Add Noise, a slight Gaussian Blur makes it form round clumps.
3. Keep it nearly colorless. Use monochromatic noise, or a faint color only in the larger clumps. Pixel-sized color speckle is the clearest sign of noise.
4. Put it under the grade. Film’s grain is in the negative, and the print’s curve shapes it: strongest in the mid tones, gone in white. Grain over the finished picture is the same everywhere. Add it before the tone curve, or mask it out of the highlights.
5. Match it to the light. A bright, sharp scene on slow film has little grain; a dim room on fast film pushed a stop has a lot, coarser in the shadows.
6. On video, let it move. Each frame of a film is new film, so real grain changes every frame. A still overlay over moving pictures is frozen.

To do it with a layer, make an overlay at the photo’s own pixel size with the film grain overlay tool, which can also grain your photo directly the way film does. The film look guide puts grain with the rest of what makes a photo look like film.

06 · Clack

How Clack draws grain

Clack is a film camera app for Android. Its grain is drawn too, on the phone’s frame, but drawn as part of each film rather than laid over the photo: the finder shows it live and the saved JPEG has it.

In film’s order

The grain goes in before the film’s curve, so the curve shapes it as a print does: strongest in the mid tones, gone in white. In the shadows it is drawn 1.8 times larger, and each dye layer has faint clumps of its own, blue the most.

35 mm’s size

Round clumps half a pixel in radius on the 12 MP still, at random, with no grid: 35 mm film’s scale, not visible specks.

Each film’s own

Portra 400 is the reference. Gold 200 is 1.27 times as grainy, from Kodak’s index; Ektar 100 a third of Portra’s, which its steeper print brings up to about half as visible, as the index has it; Tri-X 400 twice (Kodak calls its granularity of 17 fine). HP5 Plus has no published figure, so it has four fifths of Tri-X’s, a call on reviewers’ reports, not a measurement. The films with no Print Grain Index (Superia, Kodachrome, Velvia, Vision3) keep Portra’s.

Follows the light

A frame the camera had to amplify is film shot under and pushed: grain grows 1.15 times a stop of ISO above 100, up to 1.74 times at ISO 1600 and above. Rate the film under on the lab slip and it grows the same way; rate a color negative over and it gets finer; push it and the steeper curve brings it up. The slip’s Grain row says the result in words Kodak uses, from Extremely fine to Coarse. It is not a control.

New every frame

Grain changes with every frame in the finder and in video, as it does on a moving strip of film.

The film base, where the lab slip lives, runs on Android phones whose camera accepts a tone curve of its own; there, film stills ask the phone for no sharpening and no noise reduction, so the grain is the texture. On other phones Clack’s films, grain included, run on the phone’s own frame, smoothed as the phone smooths it. Tested mostly on the CMF Phone 2 Pro.

AndroidComing soon to Google Play. More about Clack