ISO is a measurement, EI is a decision: pushing and pulling

Two numbers get confused at every camera counter. ISO speed is a measurement made in a laboratory to a published method. Exposure index, EI, is the number you set on the meter. They are often the same. When they differ by more than the film's latitude covers, the development has to change to suit, and that is pushing or pulling.

Speed is a measurement

ISO 6 is the standard for measuring the speed of a black and white negative film sensitometrically: a stepped exposure, development to a specified contrast, and a speed read from a defined density point low on the curve ¹. Colour negative film has its own method, ISO 5800 ⁵, which is why Kodak can describe Portra 400 as having "true ISO 400 speed" ⁶. The method ties the number to the shadows, and the makers' own EI ranges show how much room sits above it.

EI is a decision

Ilford rates HP5 Plus at ISO 400 and then, in the same sheet, says good image quality will be obtained at meter settings from EI 400 to EI 3200, adding that this range "is based on a practical evaluation of film speed and is not based on foot speed" as the standard is ². That sentence is the licence to push. You set EI 1600, you underexpose by two stops relative to the measured speed, and you develop longer so the midtones and highlights climb back up the curve. The shadows that received too little light to form a latent image do not come back; nothing in the developer can develop what was never exposed. A pushed negative is a negative with thinner shadows and more contrast, and apparent grain usually grows with the extra development, though Kodak's P3200 at EI 1600 is a cited exception ⁴.

The tables

Ilford's figures for HP5 Plus at 20 °C, with its own DD-X at 1+4: 9 minutes at EI 400, 10 at 800, 13 at 1600 and 20 at 3200 ². In ID-11 at full strength: 7½, 10½ and 14 minutes for EI 400, 800 and 1600 ². In Kodak XTOL at full strength: 8, 11, 14 and 19 ². Notice, in the DD-X column, that the first stop costs one extra minute and the last costs seven ². Tri-X 400 is, in Kodak's words, "recommended for push-processing applications" ⁸; its push times are in Kodak's sheet.

The films that say 3200

Neither of the two fast black and white films is ISO 3200. Ilford's sheet for Delta 3200 says the ISO speed, measured in ID-11, is 1000; the recommended meter setting is EI 3200, good results run from EI 400 to EI 6400, and with DD-X or Microphen it can be rated as high as EI 25000 ³. Kodak's sheet for T-Max P3200 says the nominal speed is EI 1000 in T-Max developers or EI 800 in other Kodak developers, determined in the manner of the ISO standards and rounded to 800 for convenience ⁴. Both films are designed to be pushed, and Kodak makes a specific claim worth testing: at EI 3200 or 6400, P3200 gives better shadow detail and highlight separation than a 400-speed film pushed three stops ⁴. At EI 1600 with its adjusted development, Kodak says, the grain of the final print does not change compared with the nominal rating, only a slight loss of shadow detail ⁴.

Pulling

Pulling is the mirror: overexpose, develop less. It lowers contrast and fills shadows, which is why landscape and portrait photographers in hard light do it. Ilford's sheet carries a table for HP5 Plus "inadvertently exposed" at EI 50, 100 and 200, with Perceptol at full strength for 9, 9 and 11 minutes, and the honest note that the negatives will not be as good as conventionally processed ones ². Some films need no pull at all for a stop or two: Foma says Fomapan 100 gives good results from one stop over to two stops under with no change in development ⁷.

Tell the tank, or tell the lab

A pushed roll looks like any other roll. Write the EI on the cassette with a marker the moment you rewind, and if a lab develops it, write it on the order and ask how the lab handles push and pull and what it charges. If you process at home with CineStill's monobath, its sheet says all films can be pushed or pulled by adjusting the temperature, 6 °C per stop, warmer to push, and that above 28 °C negatives come out pushed whether you meant it or not; read the sheet for its limits before trying it ⁹. Gloves, eye protection and ventilation as usual, and the used monobath, which carries fixer and silver, is collected for disposal under local rules rather than drained.

A test worth a roll

Shoot the same scene at EI 400, 800, 1600 and 3200 and develop the roll for the EI 1600 time. Every frame gets the same development, so what you see is what exposure alone does at a fixed push: which shadows survive two stops under, and how far the highlights climb when a frame was given more. For the full picture, repeat the roll at the EI 800 and EI 3200 times and compare the strips. That one roll teaches more than any table, including these. Then read what happens when the shutter stays open for seconds, because the speed changes again.