No, it has no exposure constant.
Originally Posted by markbarendt
The edge of the cliff is the fractional gradient point. That is why it was chosen as the speed point. Once again, just because speed is calculated from the speed point doesn't mean that the shadow exposure is designed to fall there. That's where the exposure constant comes in. Post #23 and #25 on the thread ISO Speed Determination Constants begins to discuss this issue. I've also offered to email Simple Method for Approximating the Fractional Gradient Speed of Photographic Materials to anyone interested. Plus in thread #80, I've uploaded a pdf copy of Safety Factors in Camera Exposure if anyone is interested on exposure placement in regards to film speed. Thread #81 has a copy of Calibration Levels for Film and Exposure Devices if you want to know about the actual relationship between the metered exposure and speed point.
Flare is less of an issue. The illuminance range of the camera image has increased from approximately 1.50 to 1.80. But it is still a factor and many discrepancies between methodologies or confusion over numbers not matching up is do to flare.
Film speed is based on the first excellent print test. It showed that additional exposure over the first excellent print point showed no significant change in quality over a number of stops (large format). Adding 1/3 to 1 stop, therefore, isn't a big deal. Remember, film speeds were a stop slower before 1960.
The Zone System doesn't have any great insight into the photographic process. It's just that many people aren't familiar with anything else. The Zone System is just a simplified version of tone reproduction theory. Under tone reproduction the statistically average scene luminance range is 2.20. This is based on diffused highlights and shadow with some detail. That doesn't mean it ignores accent black and specular highlights. These simply print outside the LER range of the paper. Adams isn't much different. Anyone remember the aim density range in the testing section? It's from Zone I to Zone VIII, seven stops.
tone reproduction curve - range.jpg
And I'm sorry Ian, you really should support such claims.
Last edited by Stephen Benskin; 03-10-2013 at 11:30 AM. Click to view previous post history.