Quote Originally Posted by Bill Burk View Post
I've just come to realize that EI is meant to correlate your light meter to a study "Picture Tests".

A group of people were told to pick their favorite prints out of a pile.

The idea was to come up with a number to set on the meter that will make most of your prints end up in the "good" stack.
That's like how I work when I have to use a new film or developer and I'm in a hurry. There is a "standard" front lit scene every mid-morning near where I live. Direct sunlight is very consistent all year round because I'm near the Tropic of Capricorn.

I expose 4 sheets of 4x5 film in my camera with a stepped exposure sequence across each one done by pushing in the darkslide a little bit between clicks. I go from two stops under to two stops over in five steps. Because my darkroom is closeby I immediately develop all four sheets at different developing times. Within an hour I know the correct exposure for the scene. All I have to do is set a number on my light meter so that my usual metering strategy delivers the exposure reading I already know is correct. That's my film EI. And this test procedure also gives me the right development time either directly or by interpolation.

Certainly this is not a laboratory standard test but it is quick and the directness of it inspires confidence.