I was recently reading a description of the process for determining personal exposure index for various film. It seemed complicated, and I wondered why I could not merely do this:
- meter zone five on a zone card and set the appropriate exposure using the film's standard ASA
- photograph the zone card
- process normally
Then, if zone five was the center of the spectrum, you would assume your PEI matched the film rating. If zone five was shifted one or two stops in either direction, you would re-rate the film accordingly.
I don't know much about this sort of thing, so I'm sure its not that simple, but I cannot figure out why...