The BTZS book does say to adjust exposure by trial and error, but then of course the result is entirely uncalibrated wrt true speed. I'm trying to avoid that. I had another poke through the previous thread, which confirms that AV0 = f/1 = 2.5 lux-s.
I had a read of Wikipedia and it also gives the same S=0.8/Hm formula as you, which is probably (in conjunction with AV0 = 2.5 lux-s) what I need.
For ISO100 we want Hm=0.008 lux-s for the speed point, plus 9 stops so that it occurs at step 18 => 4.096 lux-s = AV0.71. So my meter would in theory read f/1+0.7 at the film plane, which is about 2 stops less light than BTZS is suggesting that I use. Any idea why the discrepancy?
PS I'm sure you're meant to take logH from lux not millilux, so the speed point is logHm = -2.1 not 0.9. Doing the computation like this in D instead of AV, we have (step 18) D=2.71, logHm = -2.1, logH = 0.61 => 4.096 lux-s as previously. See also the diagram in that Wiki page that has the speed point at about -2.4 therefore being ISO200.
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.
Edit: woah, delayed my posting and there's lots of replies...
The P=8/ISO is probably what I was looking for and closes the loop for me. That means that Hp=Hm+1.0 and therefore (ignoring flare) the speed point should be about 3.3 stops below the metered reading in a scene.
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? 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).
thanks guys. 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.
Last edited by polyglot; 10-30-2012 at 11:06 PM. Click to view previous post history.