Not with any confidence. My guess, 0.10 is about 0.30 log-H above the fractional gradient point for normal and with 0.30 added to 1.30, it equals approximately what the average luminance range was with the flare from uncoated lenses.
Originally Posted by Michael R 1974
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
Thanks, Stephen. I will continue reading. It is true I find myself looking for flaws in speed methods without having nearly enough information (or expertise). I need to keep that tendency in check.
This is from The Evaluation of Negative Film Speeds in Terms of Print Quality. Jones speculates about luminance ranges outside of average. “It is quite possible that for lower and higher brightness contrasts the value of K (ratio of speed point gradient to average gradient) will be somewhat different from that derived for the luminance range used in this work. If this proves to be true, it simply means that the effective camera speed of a given photographic negative material will be dependent to some extent upon the contrast of the object being photographed. This will mean that the photographic material in question does not have a single, unique speed value and the complete information relative to this characteristic of the material could only be given in terms of a series of numbers, or perhaps a graphic representation of the speed number of object contrast. Such a course seems quite impossible to follow, for it is necessary, in many cases, to give a single number which represents the effective speed of the material.”
In an footnote to this he states, “Since this manuscript was prepared, further statistical judgment of the minimum negative exposure required to give an excellent print when using object contrasts much lower and much higher than the luminance contrast of the test used in this work indicates quite conclusively that the minimum useful gradient is not to any appreciable extend dependent upon the object contrast. It appears, therefore, that the sensitometric criterion suggested will apply satisfactorily to a relatively wide range of object contrasts. The detailed results of this further investigation will be published shortly.”
Jones describes two problems to consider when choosing how to approach the test negatives. One is the changing lighting conditions over the period of time require to shoot a large number of negatives of different emulsions and over a range of exposures. And two, the spectral response of the different emulsions to the colors in the scene. In order to have the results of the judging relate only to the variables being tested, each negative has to be identical in both the illuminance of the scene and the spectral response.
I’m hesitant to bring this up because it will might distract from the point, but it’s necessary toward the understanding of the testing. Their approach not only solved the two problems but also provided a way to make very precise measurements. They shot the original scenes on black and white transparency film and make internegs from the transparencies. This way each negative was made from an unchanging scene without any issues of color.
Without going into detail, they took great pains to produce the internegs. From the transparency, they were able to make test series with variations in exposure and development. Here is an example of data from one of the test subjects, the famous Willow Pond. The subject had an average luminance range and consisted of many frequently encountered elements. The highlighted column shows the all the different average gradients. The column K is the gradient value at the print judgment speed point (first excellent print). The value of K is rather consistent and appears to be independent of the processing.
There were also tests with original scenes of shorter and longer luminance ranges. In A Study of Various Sensitometric Criteria of Negative Film Speeds, Jones discusses the results from a scene with a longer luminance range. “In making the series 9 to 12, a test object having considerably higher contrast was employed. This is designated as the “Woods Scene.” The brightness contrast of this case was 100. Here again the same four negative materials were employed. Again the values of K agree fairly well with those previously found, the average being 0.3196."
Last edited by Stephen Benskin; 03-13-2013 at 01:32 AM. Click to view previous post history.
Looks to me like K = Gmin/Gbar. Or am I misunderstanding something?
Originally Posted by Stephen Benskin
Anyway, do you know if Gbar was measured over the range of the scene, or if it was a fixed 1.5 in log H units?
BTW, thanks for digging up and posting all this info.
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That's the equation and that's how 0.3 was determined. Gmin is what Jones calls the limiting gradient and it does represent the first excellent print speed point or print judgment speed. It's point M in the curve.
Originally Posted by albada
Jones says Gbar is the average gradient of the used portion of the characteristic curve. Later in the paper he does define it as 1.50.
After 86 posts, we still haven't posted exactly what Delta-X is and how it, the ISO standard, and the fractional gradient relate.
The contrast parameters of the ISO speed standard provides the ΔD variable in the Delta-X criterion. The log-H range from the speed point is 1.30 but from the fractional gradient point is Δ1.50, which is what it is in the fractional gradient method. So, the ISO speed standard is in essence a single fractional gradient situation. The fixed density of 0.10 over Fb+f always has a Δ0.29 from the fractional gradient speed point. It stands in as an easy to find substitute for the trial and error method of determining the shadow gradient of the fractional gradient method. Because the ΔX value is known and unchanging under the ISO contrast parameters, use of the Delta-X equation isn't required. It's built in. If the processing conditions do happen to vary, only then is it necessary to use the equation.
Once again, the methodology is only important in determining the speed point that most closely correlates to the print judgment speed first determined in the first excellent print tests by Jones. Film speed and/or EI is then calculated from the speed point using a speed constant which is based on the desired placement of the luminance values in conjunction with the exposure meter. Exposure meters are designed to place the metered exposure at 8/ISO. Knowing the ratio between the speed point and the metered exposure point creates an understanding of where the average exposure will fall which is how the speed constant is determined. The value of the constant can be the one used in the ISO standard, or an equivalent one with the Delta-X Criterion, or one that is personalized. But whatever the it is, all are based on the same characteristic curve criteria.
Thanks for this. I will be going through your latest posts over the weekend.
Without taking away from anything else...
Originally Posted by Stephen Benskin
Is this an approximate Δ1.50? Or is a different "upper density" endpoint on logH used for the range used for the two methods? Adding 0.29 to 1.30 gives me 1.59.
Damn math! Sorry, I got a little carried away trying to make the connection. How about almost identical to the fractional gradient method.
Originally Posted by Bill Burk