Slowest ISO today?

[ac12]

Think of the streaky wispy look of slow exposure water, that is one use for SLOW films.
At ASA 25 that is 3 stops slower than my dSLRs min of ISO 200, so I can get streaks easier without ND filters than with my dSLR.

 

[petrochemist]

There are glass plates made for large format cameras that are supposed to be equivalent to ISO2, that's the slowest I marketed that I know off. Other early processes are often slower than that but would have to be self prepared.

 

[star camera company]

Wet plate is roughly 0.5 ISO. I’ve shot .8 ISO direct positive BW 35mm. Film Project has a wide range of low ISO