Bill, the problem with having the CI 0.75 curve fit the ISO triangle is that it is way too high. Long toed curves do require a slightly higher CI to fit the ISO triangle, but not this much. So, what is happening here? The testing under the ISO standard and the determination of the ISO triangle is done under no flare conditions. Rafal's test consisted of sticking the step tablet up against the window and shooting it with his camera (optical system). He emphasized the need for testing under daylight conditions but missed that such a set-up will produce flare. As we know flare compresses the shadow area. ISO speed is determined using the gradient of the shadow. CI is determined more from a fuller range of the film. In order to raise the gradient of the compressed shadows sufficient enough to fit into the ISO triangle, the CI will be excessive like we see with Rafal's examples.
What is happening here is that a non-flare interpretation is being used with a test that incorporated flare. Any results are questionable at best.
Last edited by Stephen Benskin; 08-28-2012 at 07:26 PM. Click to view previous post history.