Kodak Kodachrome 25

Before ISO 25 sounds impractical, consider what it bought. Kodachrome 25 had the finest grain structure of any color slide film ever made available to the public. National Geographic editors consistently preferred it over Kodachrome 64 for any assignment where the camera was going on a tripod or the subject was stationary. The sharpness difference between the two was visible at normal print sizes, not just in loupe examination.

The chemistry was the same K-14 dye-coupling process as every other Kodachrome: dyes added during development rather than baked into the emulsion layers, which is why the archival stability was so far ahead of anything chromogenic. A properly stored KM25 slide from the early 1980s shows no color shift. The reds are still reds. The dye structure that made this possible also meant that only Kodak's own processing facilities could develop it correctly; the K-14 chemistry was licensed to perhaps a dozen labs worldwide at peak, then progressively fewer until 2010 when it ended entirely.

At ISO 25 in bright sun, the working conditions are pleasant. A 50mm lens at f/8 and 1/250 sec is a normal sunny-sixteen exposure. Studio photographers using strobe had no speed problem. The film was poorly suited to available-light work or anything requiring handheld shots indoors, and photojournalists who needed to shoot fast generally used Kodachrome 64 or shifted to faster Ektachrome variants when the light dropped.

Kodak discontinued Kodachrome 25 in 2002, eight years before the K-14 process shutdown ended the entire Kodachrome line. The K25 emulsion itself dates from 1974, when the K-14 process replaced K-12 and Kodachrome 25 took over from the older Kodachrome II at the same ASA 25 speed. Existing stock can only be processed as black and white now. Zone Light Meter lists it for that use case, with reciprocity exponent 1.10 applied past one second for long exposures on recovered rolls.