The diagram on the left is from ISO Ė 6 Black and white pictorial still camera negative film / process systems Ė Determination of ISO speed. While it doesnít say in the standard, the reason why the specific contrast parameters are used is because they are part of an equation that links the fixed density method of speed determination with the fractional gradient method. The diagram on the right shows this relationship.
Itís possible to find the fractional gradient speed from any contrast simply by plugging the negative density range at 1.30 log-H units from the speed point into the Delta-X equation. The reason why a density range of 0.80 was chosen is because of the relationship between the speed point / metered exposure ratio and the average scene luminance range (See What is the Relationship Between Film Speed and the Exposure Meter thread).
When the ISO contrast parameters are met, itís not necessary to do the actual calculation to find the value for Delta X. Because the equation isnít required to determine the ISO Delta X value, itís not included in the standard.
The contrast parameters of the ISO standard are about creating a good correlation between the fixed density method and the fractional gradient method. It does not represent and has nothing to do what some call ISO normal development.
I have to admit, this one is a little more hidden.
Last edited by Stephen Benskin; 01-24-2012 at 03:51 AM. Click to view previous post history.