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# Thread: Sensitometry and lens transmission formulae

1. Originally Posted by polyglot
... I'll ignore the BTZS suggestion for now and do my first exposure test (for ISO100) at 4 lux-s (f/1+0.7) instead of 16 lux-s.
Sounds right... just don't do all 5 sheets before you see where it lands.

2. Originally Posted by polyglot

Second question is "what is the offset between speed point and metered reading?". In other words, where on that H-D graph is the "average reflected reading" going to fall? Is it at n, i.e. Hn = Hm+1.3? This is basically a re-statement of my original question for this thread, trying to directly relate light readings in lux-s at subject and at film.
You've already answered this. The ratio is usually noted as n sub 1. I did a thread on this subject "What is the relationship between film speed and camera exposure?" I've also attached a paper that you might find interesting, Connelly's "Calibration Levels of Film and Exposure Devices."

Calibration Levels of Films and Exposure Devices, Connelly.pdf

Did you successfully make a test-strip at that exposure and have the speed-point land where you expected it on the test strip? What if you set the meter to ISO400, that should get it to read a larger aperture: f/1+0.7 at the correct light level (assuming the limitation is the meter's display and not its sensitivity).
My sensitometer is (or at least was) calibrated. I did use those Log-H equations to determine which step tablet density I wanted the speed point to fall. Worked perfectly.

3. Cheers for the PDF.

4. Originally Posted by polyglot
As for f/0.7 for ISO400, that agrees well with these calculations rather than the BTZS suggestion. Hm = 0.002 lux-s plus 9 stops = 1.024 lux-s = -1.29AV, which is 1/3 stop less than f/0.7; or f/0.5+0.7 if your meter will read f/0.5 Did you successfully make a test-strip at that exposure and have the speed-point land where you expected it on the test strip?
f/0.7 proved to be too high exposure, and I had not enough of a toe in my tests. I estimated a new exposure, about 1.5 f lower, and that worked well for HP5+. I fudged it with 320TXP, which needed a little less exposure, about 0.5f, but I dialled down too much, and ended up having pretty toe but no DMax for the 4, 5.5, and 8 minute dev times.

I second Bill's suggestion to shoot just one sheet to test exposure. You can develop it quickly in a tray, even if the main test uses a tank etc. That's what is holding me back from doing it once more (3rd time!) for 320TXP: needing to do 5 tank-loads of several sheets, as I am lead to believe I would get incorrect results if doing less than the full load each time (I usually do 5 or 6 sheets at a time). Also, all of this testing is getting film-cost pricey.

5. Originally Posted by Stephen Benskin

Finding log-H
log((1 / antilog Step tablet density) * Illuminance) = log of transmitted light or log-H
..............................
Also, note you can convert transmittance, density, illuminance etc to logs then you can add and subtract rather than divide and multiply. So the exposure to your film at the speed point is the sum of the log of step density, log of additional ND over the light and the log of the lamp's meter-candle-second (or lux-seconds)

6. Here's a graph showing the standard model of how the scene luminance fits on the characteristic curve. The log-H values are for a 125 speed film.

7. Originally Posted by Stephen Benskin
Here's a graph showing the standard model of how the scene luminance fits on the characteristic curve. The log-H values are for a 125 speed film.

Metered exposure to ISO speed point = 1.0 log H? Not 1.2?

8. Originally Posted by ic-racer
Metered exposure to ISO speed point = 1.0 log H? Not 1.2?
That's what you'd get from the ratio of Hm=0.8/ISO and P=8/ISO. Not that I have any citation/justification for the latter formula other than posts in this thread.

9. Originally Posted by ic-racer
Metered exposure to ISO speed point = 1.0 log H? Not 1.2?
I'm surprised you don't remember either thread. The 1.0 ratio is the reason why there's a consistent discrepancy between ISO speeds and Zone System EIs.

The proof for the ratio is with exposure meter calibration and how it relates to the metered camera exposure point.

10. Originally Posted by Stephen Benskin
I'm surprised you don't remember either thread. The 1.0 ratio is the reason why there's a consistent discrepancy between ISO speeds and Zone System EIs.

The proof for the ratio is with exposure meter calibration and how it relates to the metered camera exposure point.
I see how this discrepancy (between 1.2 Zone System and 1.0 ISO) can explain 2/3 stop (of the 1 stop typical) discrepancy between ISO and Zone System speeds...

Zone System: Meter Zone V, stop down once Zone IV (0.3), twice Zone III (0.6), three times Zone II (0.9), four times Zone I (1.2)... where you expect 0.1 density speed point to exist on Zone I (four stops below metered point)...

versus

ISO: Meter, stop down 1 stop (0.3), 2 stops (0.6), 3 stops (0.9), 1/3 more stop (1.0)... where you expect 0.1 density speed point to exist at 3 1/3 stops below metered point.

Tell me... That's why I shoot TMY2 at 250 instead of 400... It's that simple?

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