Quote:
Originally Posted by Peter Hogan
It's a fact that using rated ISO figures and recommended dev. times [B]will[B] result in overexposed and overdeveloped negs. Using standard developers you really need to downrate your film by as much as 30% to 50% and reduce dev times by a similar amount.
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Hmmm,
I think this is not necessarily true. The question is how many stops of subject range do you expect to capture on film? Paper holds 7ish. Film manufacturers tend to give speed and dev times to capture 7ish stops on neg film.
Now if you are a zone system person you will no doubt think you need 10 stops but that is not true for a lot of people and a lot subjects.
If you assume 1 zone is one stop and then apply it to a manufacturers dev time for 7ish stops then you are making a mistake. If the film dev captures 7 stops then divide 7 by 11 ( 11 zones. 0 thru X) which gives 0.63 and then each zone is 0.63 stops.
So if you then meter a zone 3 value you close down 1.26 stops and not 2. You would be 0.75 of a stop out if you used 2 stops. And if you meter a zone 8 value, then you open up 2 stops and not 3. You would be a stop out if you used 3. And that is where the myth comes from that film manufactuers speeds are wrong. It is zone system workers who think AA's 1zone = 1stop is a fixed rule. It isn't but they try to apply it to a neg range which it doesn't fit. Then they tailor development to fit the zone system and to do that they need more exposure and less development. But that doesn't mean manufacturers speed or dev times are wrong.
Then the question is how many stops in the subject and how many can I get on the film without making the subject look flat because it's compressed too much. Most closed (no sky) subjects are 7 or less stops. Only when you start metering bright clouds and deep shadows do you start getting the 10 stop and plus ranges.