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I'm not set up to post visual content right now. The same thing happens if a long densitometer plot is scrunched to fit a small graph, versus a short line stretched to fit the same space. It's not an apples to apples comparison.
So the answer is that Ilford for reasons known only to itself has scrunched up two long densitometer plots of for Pan F and FP4+ to fit a small graph and in so doing the FP4 graph exhibits the S curve but the Pan F graph scrunched the same way over presumably the same scale does not.
I can't really work out how scrunching up both curves over the same "long plot" results in the FP4+ curve demonstrating the S shape but not the Pan F?
If this is all making sense to others then please join in and say why two long densitometer plots scrunched into a much smaller graph results in two different graphs.
pentaxuser
It doesn't help when the curves are published so small. But ID-11 graphs are available in the older tech sheets, which would replicate common D76 results too, but still be no substitute for a family of curves. Otherwise, I'm running out of patience with certain people who don't understand the basics. It's like reading music. It wouldn't matter if I posted a hundred plots if they don't know what the notes mean.
As mentioned earlier in Post #115 , scaling is the same for the Ilford plots (i.e., the domain and range, as well as the units). The film developer was the same too. If Ilford's plots are not an apples-to apples comparison, nothing is. The curve shapes may change with another developer -- this would be an orange-to-orange comparison.
I don't have access to anything like a flatbed scanner. Left all that behind when I retired. Do all my own plots on official Kodak log paper. If anything represents a common denominator standard bridging multiple decades, that does. Tutorials and books are readily available. All the Kodak b&w film guides, often found in used bookstores, contain an introductory section explaining characteristic curves. A more involved paperback primer would be Sensitometry for Photographers by Eggleston. The problem with small graphs is that people do not ordinarily perceive the impact of what seems to them to be very minor differences, which in fact are significant because they are logarithmic, and would be more obvious on larger scale. That's particularly the case with toe profiles. The original Tech Sheets for current plus-version FP4 isn't even dated, but is based on ID11, ironically not even plotted to the point of the shoulder. But the distinction from Pan F is quite evident nonetheless. Pan F has a significantly longer toe, as well as a harsh shoulder beginning to be evident even in their kind of truncated presentation - in other words, an S curve, apples to apples. It takes quite a bit more exposure relative to rated speed for FP4 to shoulder off, though that characteristic would appear in a longer scale plot. A bigger graph would also allow one to subdivide sections into smaller increments and more easily detect what's really going on. With old fashioned plotting paper, you've got ten times as much grid detail.
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