I just covered this in the new thread by Bill. More information is in the thread What is the Relationship Between Camera Exposure and Film Speed. Basically all film speed does is to produce an index number useful for calculating camera settings. Connelly’s Calibration Levels for Film and Exposure Devices covers all of this.2. In your paper you state the ISO speed point falls 3 1/3 stops below the meter calibration point and shadow exposure falls 4 1/3 stops below. I’m confused by both these intervals.
Calibration Levels of Films and Exposure Devices, Connelly.pdf
Michael, all sorts of subjects were photographed and printed on multiple grades. Of course they emphasis “normal” conditions. BTW these tests also determined what are considered normal conditions. I have to refresh my memory as to the details, so I’ll have to come back to this point. In Minimum Useful Gradient as a Criteria of Photographic Speed, Jones writes, “The one factor of fundamental importance which determines the quality of a picture made by the photographic process, whether it be a print on paper or an image projected on a screen, is the relation between differences as they exist in the original object and the corresponding luminance differences in the reproduction. Therefore, from the standpoint of tone reproduction and this is the most vital consideration in judging the quality of photographic results, the gradient characteristics of the negative and the positive materials are of primary importance.”It occurs to me, the print evaluation studies originally used in deriving 0.3G involved exposures of average scenes. So, to me, it would mean in the context of the tests, developing the films to lower gradients (for example) would have constituted “errors” to be corrected with an increase in paper grade – rather than deliberate attempts to accommodate a higher than normal subject luminance range. This could explain why I was initially startled by the six graphs on p.325. Before going into this, I had actually expected at least some of the print judgment speeds and 0.3G speeds to be lower than the fixed density speeds (eg: the long toe, low gradient curves). But this is the opposite of what was concluded in the print evaluation studies. While the math made sense to me, intuitively the graphs just didn’t look right – unless one simply increased the paper grade to effectively normalize say the bottom two curves in Fig.2. This is my take on why the fractional gradient method is not more concerned about a precise contrast. A constant relationship of 0.3G seems to work in print judgment tests, apparently regardless of how low the absolute toe contrast is, because a correction via increase in paper grade would bring both the toe G and average G up. I think this is sort of a different way of saying the same thing as the possible explanation you outline in your paper for why fractional gradient is not more concerned with a target contrast, and why the Delta-X film speed stays relatively constant with deviations in delta D.
The examples are not about log-H but about the consistency different film speed methods have with the print judgment speed. Since the print judgment methodology isn’t practical, another has to be chosen. If the 0.10 fixed density method tends to fall 0.26 log-H units to the right of the print judgment speed point, it would be an effective substitute only if it always falls 0.26 log-H units to the right. This is why it’s important to test each methodology using films with different shaped curves and under different processing conditions.
If the 0.26 difference was consistent and the desired placement of the shadow exposure was at the print judgment point, all that is necessary is to divide the log exposure at the 0.10 speed point into a constant that adjusts for the 0.26 difference. Let’s say the difference is one stop or 0.30 log-H. All that is necessary to have the exposure fall at the print judgment point while calculating the film speed from the 0.10 speed point is for the equation to be 2 / Hm instead of 1 / Hm.
“It does not seem possible that the question of how much of the underexposure region [toe] can be used can be answered in terms of density which of itself tells absolutely nothing as to the way in which luminance differences will be reproduced. When we say that this question must be answered in terms of gradient, we mean, of course, in terms of the gradient of both the negative and positive materials, since both of these factors must be considered in order to determine the relation of luminance differences in the reproduction to those of the object.” – Jones