Effective speed

The effective speed of film is the point on the film curve where density reaches 0.1 (density units) above the density of film base and (normally occurring) fog. Because shadow areas develop out very quickly, the particular developer used makes no difference. 0.1>B+F is Zone I in the zone system. This is usually one half to one full stop slower than the manufactures' rating. If you lack access to a transmission densitometer, you'll be safe to divide the speed by 2 and set your meter accordingly. Make sure your shutter is calibrated accurately. There are proceedures for arriving at your personal film speed and development times clearly documented in Ansel Adams book The Negative and in Fred Picker's book The Zone System Workshop.

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