Thanks for the message! I suspected David had worked up some pretty smooth logic so we need not fiddle different film speeds when using sheet film.
Let me make sure I understand what is going on here.. We all started out by adjusting enlarger height and aperature at EV4 (meter at 100, no cone) for 400 ASA film.. We then made our step tablet exposures at 4M, 5.5M, 8M, 11M and 16M. From those test step tablets, we measured our densities and loaded them into Plotter. Based on our already loaded test data, we should be able to look at the X-axis of the film curves and see how far off from a nominal 2.4 our speed points are, right? If memory serves, the only tweaking we did when making our original film tests was to make modest adjustments to enlarger height IF we had too many or too few blank steps. If I understand, your article on determining speed point is, in fact, measuring the impact on the exposure curves of those small adjustments we may have made during our original film tests?
On the issue of how to use speed points, let me pose the question another way, but first I'll sketch out what has been done so far. Not having the test exposure box for use with 35mm or 120 film, Fred Newman was kind enough to expose 120 format TMY, PAN-F, D100, D400 and NEOPAN100 for me. I run all my 120 and 35mm in a JOBO processor rather than just a simple hand tank. Obviously, I cannot optimize exposure and developing times for each frame, but I can at least tinker with developing time for the entire roll. Assuming I have a normal SBR range of ~7 stops for a given roll of film, I tend to use my test data to determine developing time for the contrast range I think I might want (for "normal scenes" I aim for a g-bar of around 0.54-0.56 - this number comes from Koadk Pu F-9 for Tri-X Pan film where they explain 0.56 is the CI for "normal developing").
looking at my test data (my D76 1:1 chemistries, my JOBO, my own operational biases), and (for now) selecting a nominal 2.4 speedpoint, I see that TMY, DELTA 400, DELTA 100, NEOPAN 100 and PAN-F have the following characteristics:
*** ALL DATA BELOW FOR D76 1:1 at 70F, JOBO at 75rpm) *** FOR TMY 4M: 250++ speed, G-bar 0.27, SBR 13.1 5.5M: 320--speed, G-bar 0.34, SBR, 10.2 8M: 320- speed, G-bar 0.43, SBR 8.2 11M: 320+ speed, G-bar 0.52, SBR 6.7 16M: 400-- speed, G-bar 0.65, SBR 5.4
FOR DELTA-400 4M: ISO 400 speed, G-bar 0.34, SBR 9.8 5.5M: ISO 400 speed, G-bar 0.37, SBR, 8.9 8M: ISO 400 speed, G-bar 0.44, SBR 7.5 11M: ISO 400 speed, G-bar 0.52, SBR 6.4 16M: ISO 400 speed, G-bar 0.61, SBR 5.4
I also have similar data for DDX 1:7 at 70F in the JOBO at 75rpm... I'll post it later if people are interested.
For example, I see from my TMY data, that the film can handle 13 stops of range with a short 4M development time. Under these conditions, the film would have a speed of 250++. At the other end of the spectrum, for a 5.5 stop range and a 16 M development time, the film would have a speed of 400--.
So, looking at all this stuff and wondering how to take some of the excellent sheet film/BTZS knowledge and transport it roll-film land, the question is how to use the speed and developing data.
The Plotter SW tells us certain film characteristics arising from testing at EV 4 light level (meter at 100 ASA, no cone) for a vendor rated 400 ASA film. When we meter in the field using BTZS method, we set the incident meter at 100, NOT rated film speed.
To carry on in roll-film land, I think things are less clear. Since so many roll-film cameras now perform automatic exposure calculation, I am presuming that one would use our CALCULATED film speed after setting our speed point in order to get optimal exposure and develop for a time period sufficient to generate the CI one seeks?? Or should we not horse around and simply leave the ASA setting on the roll-film camera set at the manufacturer's rating and develop for our target CI??