Your example provides an interesting problem for the ISO standard, "how do I get it to describe what my mid-tones will do in a print?" In a simplified sense, it can't, the question the ISO standard and other shadow point methods answer is just "what the reasonable lower limit?" We still have to figure out how the rest of the film behaves.
If the "print reference point" is the "metering point" and that was simply a density of .4 or .45, something appropriate, we might talk about film characteristics differently. TX might still get a 400 rating and TXP might get 200, so what, .4 may very well be a better predictor of print results.
In a broader sense, the delta-X method relates the speed point (essentially the basis for shadow point methods) to the fractional gradient method - which was based on print studies, not arbitrary shadow placements.
My two cents.
How will Delta-X change the example? It wouldn't. The ISO standard uses Delta-X Criterion. That's why the US agreed to the change.
Apart from the greater tendency of what Jones calls "first choice prints" is more likely under certain printing conditions, I can't see a problem attempting to peg a certain density at a certain point on the curve. As long as the photographer has customized it to their materials and personal preference. A specific target negative density; however, shouldn't be considered universally applicable. Take the two quad example. What makes it a correct exposure for the ISO is not the negative density at the metered exposure point, but that the metered exposure was at 8/ISO and the exposure fell at the correct ratio from the speed point.
In the scientific photography papers, phrases like "greater tendency" and "more likely" are used a lot. Most methodologies will work adequately. The question is always about the one which has better results most often. This doesn't mean that in certain situations another method will work better. Just that in most situations, one method tends to have a higher consistancy. The question of fixed density vs fractional gradient is about what is more accurate over the greatest number of situations. It's all one big normal distribution curve. Advocating one approach over another is only about what is more likely.
I have repeat the ISO Test with a Gray Card... Photographing with a 400 TX I have to setup the Lightmeter to ISO 200 for my personal setting ;-)
Everything is good:-)
THX for your great constructive help!
Itīs time to take pictures :-)
Is there a prefered way how the development times should be changed? I would like to retest FP4+ in 510-pyro semi-stand developed. My current development time is 18 minutes. So I was thinking to use 5 developments of resp. 5.5, 8, 11, 19 and 22 minutes. Any ideas why this is a good or bad idea?
I'd make the longest time a bit longer, so if you wish to have 19 min, make the longer one, perhaps 35 min. The idea is not to hit the exact target time with a test, but to generate enough data to the "right" and "left" of your target times that let you safely interpolate the actual, correct duration. If you feel that you are good with 18 min, and you wish to re-test, then you need enough data on both sides of the 18 min to be sure that, for example, 25 would not be better.
In general, choosing 5 dev times that follow the sqrt(2), ie, 1.4142 sequence helps to cover enough range for typical N-2 to N+2 needs. If your 18 min seems to be your N time, then I'd suggest a sequence of: 9, 12, 18, 25, 36 min. The key to the test is the interpolation of the correct times in-between the tested ones, based on the curves/contrasts that you would get from those tests, and those that you would desire to get. Ralph's method will lead you there, or you could follow many others. I've published curve-plotting software on this forum, too, which can also calculate the resulting times, based on your contrast requirements, but it can all be done, easily, by hand.