Michael, there are two different ways to explain how this is possible with the ISO standard. The standard's contrast parameters (ISO Triangle) is really part of an equation that creates a good correlation between the fixed density method and the fractional gradient method. If a correlation didn't exist, I doubt the fixed density method would have been adopted. What this means is while the speed point is at a density of 0.10 over Fb+f, when the parameters are met, the factional gradient speed point will always fall approximately one stop to the right. As we all know, the fractional gradient point is the minimum point of exposure where a print considered of excellent quality can be produced from the negative.
Originally Posted by Michael R 1974
This except from Safety Factors in Camera Exposure by C.N. Nelson explains the reasoning.
"Proposed Change in Speed Criterion
The reduction in the safety factor could be accomplished simply by changing the constant in the ASA formula for deriving the ASA exposure index from the ASA fractional-gradient speed of the film. The present formula, which gives a safety factor of about 2.4, is
Exposure Index = Fractional-Gradient Speed / 4Es
or Exposure Index = 1/ 4Es
where Es is the exposure in meter-candle-seconds at the fractional-gradient speed point and 1/Es is the ASA fractional-gradient speed. If the constant of ¼ were replaced by a constant of ½, a new type of "exposure index" would be obtained which would provide the proposed lower safety factor of about 1.2.
There are several reasons, however, for adopting not only a new constant but also a different speed criterion. The fractional-gradient criterion was originally chosen because it has the desirable feature of giving speeds that correlate closely with speeds obtained by practical picture tests. It has the objectionable feature, however, of being somewhat inconvenient and difficult to use. Consequently, a simpler and more convenient criterion, such as that based on a fixed density above fog density, is often desired. Fortunately, as shown by the recent data of Nelson and Simonds, "a good correlation exists between fractional-gradient speed and speeds based on a density of 0.1 above fog, provided the development condition's are controlled so that a fixed "average gradient" is obtained...Thus the adoption of the 0.1 fixed-density speed criterion in combination with a suitable development specification would offer the advantages of convenience and practical significance."
The other way to explain it is easier to picture. The ratio between the 0.10 fixed speed point and the metered exposure point is 10x or a range of 1.0 log-H. That's 3 1/3 stops. The average shadow falls 4 1/3 stops below the metered exposure point. That's a difference of a stop. The no flare curve uses a point one stop above where the shadows fall as the speed point because in practice flare will bring those shadows up.
The reason why Zone System tested EIs tend to be 1/2 to 1 stop lower than the ISO speeds comes from a misinterpretation of the above concept. As I've pointed out before, in camera testing of a single toned target will yield almost zero flare. Zone System speed point falls 4 stops down from the meter exposure point or 2/3 stops below the ISO speed point with the 10x ratio. The Zone System testing method assumes a flare test condition for a no flare test condition thus causing the speed discrepancy between ISO film speeds and Zone System EIs.
Last edited by Stephen Benskin; 08-29-2012 at 07:04 PM. Click to view previous post history.