Reading a film data sheet: rms granularity, Print Grain Index and lines per millimetre

Every film comes with a data sheet, and most photographers read only the development table. The other pages answer the questions people argue about in comment threads, how grainy, how sharp, how it holds up in a big print, with numbers measured by the people who coat the film. Three of those numbers are worth learning to read.

Rms granularity: grain measured on the negative

Kodak's black-and-white sheets give a "diffuse rms granularity", read at a net diffuse density of 1.00 through a 48-micrometre aperture at 12× magnification ¹. Think of a tiny circle of light scanning a patch of evenly exposed, developed film: the figure is how much the density flickers from spot to spot. Lower is finer.

Film	Diffuse rms granularity
Fujifilm Acros 100 II	7 5
Kodak T-Max 100	8 1
Kodak T-Max 400	10 2
Kodak Tri-X 320	16 3
Kodak Tri-X 400	17 3
Kodak T-Max P3200	18 4

Fujifilm reads Acros II through the same 48-micrometre aperture, after Microfine development ⁵, so the black-and-white figures are close to like for like. Kodak's slide film figure is taken a little differently, at a gross diffuse visual density of 1.0: Ektachrome E100 measures 8 ¹⁰. Two things stand out. Kodak calls Tri-X's 17 "fine" ³, which tells you the scale is steep; and T-Max 400 at box speed measures finer than Tri-X 320 ^2,3. Ilford publishes no rms figure for its films; its HP5 Plus sheet deals with grain by naming the developers that give the finest ¹¹.

Print Grain Index: grain measured in the print

For colour negative film Kodak stopped quoting rms and publishes a Print Grain Index instead. Its sheets are blunt about it: the index replaces rms granularity and uses a different scale that cannot be compared with it ⁶. The index is perceptual. A value of 25 is roughly the threshold at which grain becomes visible, and a change of four units is the smallest difference 90 per cent of observers can see, at a standard viewing distance of 14 inches, the distance for a 4×6 print ⁹. It assumes diffuse printing light; Kodak warns that condenser enlargers, with their more specular light, may show more grain than the index suggests ⁹.

From 35mm	4×6	8×10	16×20
Ektar 100	below 25	38	66 7
Portra 400	37	59	89 6
Portra 800	48	70	99 8

The surprise is in the formats. An 8×10 from a 35mm Portra 400 negative scores 59; the same print from 120 scores 37 ⁶. That gap of 22 units is about five just-noticeable steps, twice the difference between Portra 400 and Portra 800 at the same size ^6,8. If a big colour print is the goal, the format decision matters more than the film decision.

Resolving power: two charts, two answers

Kodak and Fujifilm give resolving power in lines per millimetre on two test charts: a high-contrast one at 1000:1 and a low-contrast one at 1.6:1. T-Max 100 resolves 200 and 63 ¹; T-Max 400, 200 and 50 ²; Acros II, 200 and 60 ⁵; T-Max P3200, 125 and 40 ⁴. The low-contrast chart is the harder test and the nearer one to soft detail in an ordinary scene, which is why the second number in each pair deserves more attention than the first.

What it is coated on

The sheets also say what holds the emulsion. Ilford coats HP5 Plus on a 0.125 mm acetate base in 35mm, 0.110 mm acetate in 120, and 0.180 mm polyester for sheet film ¹¹. Kodak's July 2025 colour sheets list a 0.10 mm ESTAR thick base, Kodak's polyester, for both 135 and 120 of Portra 400 and Ektar 100, and 0.19 mm for sheets ^6,7.

Reading your own

The conditions printed beside each figure are part of the figure: a developer, a density, an aperture, a chart contrast. Your developer and your scanner will move the numbers, which is fine; the sheet's value is that every film in a maker's range is measured the same way, so the comparisons hold.