Box speed is a measurement: what the 400, the 1000 and the 3200 really mean

The number on the box is a measurement, made under stated conditions, and a few films print one number on the box and another in the small type. Reading the small type explains why some films push gracefully and one famous film does not push at all.

The standard and the practical rating

ISO 6 is the international method for measuring the speed of black-and-white negative film ¹. It works from the foot of the characteristic curve, the faint end where density first rises out of the fog. Ilford says plainly that it does not rate its films that way: its sheets state that the ISO speed was measured with ID-11 developer at 20 °C in a practical evaluation of film speed, and is not based on foot speed as the ISO standard is ^2,6. A practical rating asks what exposure makes the best print, not where density first appears.

The 3200 films are not 3200 films

Ilford's Delta 3200 has an ISO speed rating of 1000/31° to daylight ². The 3200 is the recommended meter setting, paired with extended development, and Ilford gives good quality from EI 400 to EI 6400, with ratings up to EI 25000 possible after test exposures ². Kodak's T-Max P3200 is the same idea with a different number: its nominal speed is EI 1000 in Kodak's T-MAX developers and EI 800 in other Kodak developers ⁵. Kodak adds that above EI 6400 it is critical to make tests first ⁵.

How far the everyday films stretch

HP5 Plus is rated ISO 400, and Ilford says it makes high-quality prints at meter settings up to EI 3200 with extended development in DD-X, Ilfotec HC, Microphen or similar ³. FP4 Plus is ISO 125, with good quality from EI 50 to EI 200 without leaving the normal range ⁷. Those ranges are the maker's own: they tell you where the film was designed to be comfortable.

The film that will not push

XP2 Super is the exception that makes the rule clear. It is a chromogenic film: its image is made of dyes formed during development in ordinary C-41 colour chemistry, and its ISO 400 rating was measured in standard C-41 processing ⁴. Ilford says push processing gives no practical increase in speed, unlike conventional ISO 400 films; instead it can be exposed at EI 800 and given standard processing ⁴. Its latitude lives in the exposure, not in the developer.

So what is the speed of your film?

Whatever your negatives say it is. Box speed is the starting point measured by the maker; an exposure index is your decision, and your developer, agitation and scanner are part of it. Shoot a roll bracketed around the box speed, develop it the way you always do, and let the shadows tell you.