What a complicated answer.
Simply put, most film is designed to react to light in a linear fashion
from about 1/2 second to 1/1000 second. If times slower or faster than this
are used, the light is not recorded in the same linear fashion. Therefore
an adjustment in time is necessary.
If you want to use a film for which you will never have to add more than
1/2 stop to compensate for this lack of linearity, use Fuji Acros. It is
rated at ISO 100, but for long exposures becomes the fastest film
available. Yes, even faster than T-Max 3200. There is no correction needed
for the first 1000 seconds, and only 1/2 stop needs to be added beyond that
time.
Jim
