3M ScotchChrome 640T

ScotchChrome 640T was the obscure professional option for stage lights and tungsten-lit interiors where flash was forbidden and 400 ISO was a stop too slow. 3M and Ferrania produced it in 35mm only, sold mainly through mail order from the mid-1980s through the mid-1990s. The "T" meant tungsten balance, calibrated to 3200K bulbs. Use it under daylight and it goes blue. That is by design.

Where the film actually shone was the live theater photo pit. ScotchChrome 640T had a calibrated response to stage lighting that rendered reds, ambers, and warm whites without the saturation push that made Ektachrome 320T look strident on the same scene. Theatrical photographers used it for production stills and program covers in the late 1980s. Concert shooters reached for it where the only alternative was Ektachrome 160T pushed two stops or Fujichrome 1600D, both with their own compromises.

Grain at 640 ISO in an E-6 reversal film was substantial. Projection at 4x6 feet showed clear structure. Drum-scanned for print at 11x14, the texture was the dominant character. Cross-processing in C-41 produced the blue-yellow shift the lomography community later rediscovered.

Process in standard E-6; pushing to 1280 was supported by 3M's published times. For daylight work, an 85B amber correction filter brought the effective rating to about ISO 400 daylight, usable outside the stage context if you accepted the speed penalty.

The 35mm-only format meant medium and large format theater shooters never adopted it. ScotchChrome 640T was rebadged briefly as Imation Chrome 640T after the 1996 spin-off and then disappeared. Surviving stock is rare, well-expired, and shows pronounced base fog after thirty-plus years.

Reciprocity exponent is 1.1, applied by Zone Light Meter past one second. A metered 15-second exposure becomes roughly 19 seconds at the film. That matters less for the stage work the film was designed around, where shutter speeds rarely dropped below 1/60, and more for tripod interior shots photographers occasionally used it for.