Right now, there is a great selection of interesting ISO 100 films I would like to try, but only if I can actually use EI 100, and not 50 or 64. I know the generally accepted definition of "medium speed film" includes ISO 100, but in my personal dictionary, ISO 100 is "slow" even if metered at 100, and unacceptably so at EI 50.
Most often, I am using 35mm b&w negative film, hand-held. And most often, I prefer to use my cameras' built-in light meters, along with some common sense about where to aim the camera when metering that comes from 50 years of experience. I do carry a gray card and a decent hand-held light meter for tricky lighting conditions where I may want a second opinion.
When I try to hand-hold at 1/60th second, I have found a few of my frames will suffer from camera shake, so I try to use 1/125th second and faster. And while I sometimes choose to use apertures wider than 5.6, I want that to be my choice, and not something dictated by marginal film sensitivity.
According to the definitions given on <
this Wikipedia page>, my usual shooting conditions have EV(100) values between 12-15. If EV 15, no problem; I can choose any combination between 1/125@f16 and 1/
1000@f5.6. But if conditions are EV 12, then 1/125 requires f5.6, and any aperture smaller than 5.6 requires shutter speeds slower than 1/125 sec. So for me, metering at EI 160-200 is a more reasonable choice -- but there are only a very few b&w films with ISO ratings in that range.
Over the next few months, I will be trying some ISO 100 and 125 b&w films metered at EI 160-200 and developed in Ilford Microphen to see if I like the results. Ilford claims Microphen "gives an effective increase in film speed whilst retaining the grain characteristics associated with fine grain developers."