Diapositivo,
As I said in a previous post, I thought that reflection meters use 100 speed as a baseline speed. The reason I thought this is that my Minolta Spot meter definitely uses 100 speed as some kind of baseline because in order to get a cd/m
2 value from it, you must use speed 100 to get EV and then convert using a provided chart of numbers. But that may just be because there is only a chart for a 100 speed.
Anyway lets assume for now that a speed of 100 is a baseline for reflection meters internal processing and that the meter assumes a speedpoint of 0.1 above fb+fog. Then the reading given would produce 3 exposure stops more than the speed point requires as we determined in earlier post (16*100*0.08 = 128 = EV7). So infact 3 1/3 exposure stops above what we expect for fb+fog(0 exposure).
Well we know from the multitude of graphs produced (Bills example below is good), that using ISO speed and manufacturer recommended dev that for 3 1/3 stops of exposure we can expect a neg density of 0.6 including fb+fog for that point on B&W negative film. We also know that we require a negative density range of 1.3 for a full scale negative from a 7 1/3 stop subject brightness range. So if our subject is 7 1/3 stops range from black to white then we have 4 stops of subject brightness range above our lower 3 1/3 stops and we know that it will fit into the top 0.7log of our 1.3log full negative density range.
So that tells us that our meter, for a B&W negative film is placing the exposure 3 exposure stops up from speed point and 4 exposure stops down from 100% of the exposure range. 4 stops less than 100% is approx 6.25% of 100%.
So I conclude that if we metered a grey card we would expect it to be 3 exposure stops up from speed point which would be on a density of 0.6log of opacity including fb+fog assumming film is B&W negative, the meter is using 100 speed as baseline for its internal processing (which we don't know for sure) and manufacturers ISO speed and recommended dev are being used. And K = 12.5
The assumed meter baseline of 100 speed seems to fit what is actually happening so we'll go with that for now.
Borrowed from Bill, I hope he doesn't mind. I can remove if he does.
SLIDE FILM
check following carefully as I'm not 100% its right.
So what about slide film metered with a spot meter? Well slide film has a much steeper curve over a shorter subject brightness range so things will be different. I'll use Provia 100 as an example. It gets a bit tricky straight away becasue it depends where on the curve you pick as being black, as soon as the colour curves start to separate or somewhere else with less exposure where the curves have separated a little further. Well for the purposes of this example and using the fuji H&D curve for provia 100 I'll give 4 examples of where black may be considered:
Same meter as used for negative film which I think we are now fairly sure of how it works.
1 black is -3.0 LogH (lux seconds) = 0.001 lux seconds eposure
2 black is -2.0 LogH (lux seconds) = 0.01 lux seconds exposure
3 black is -1.0 (lux seconds) = 0.1 lux seconds exposure
4 black is -1.75 (lux seconds) = 0.178 lux seconds exposure
we just meter the subject whatever it is (maybe a grey card ) and reading from the fuji H&D curve we just look 3 + 0.1log speed point exposure stops to the right (1.0LogH shift to right) on the LogH exposure axis and read off the slide density above it so we get:
1 black is -3.0 LogH = 2.8 log slide density
2 black is -2.0 LogH = 1.0 log slide density
3 black is -1.0 LogH = 0.2 log slide density
4 black is -1.75 LogH = 0.75 log slide density
However, this is tricky because we can't judge by eye exactly what any of these dark colours are so we can't be sure where the tone will end up. And we know we must preserve the highlights on slide film.
So, what if we want a grey card to be slide density 0.75?
What we can do is meter something which we want to be just full white, this will give a reading which is shifted 1.0logH(3 1/3 stops) to right and be vastly overexposed well into the white. But knowing that we can close down by 3 1/3 stops + close down another 0.75logH (2 1/2 stops) to put our result on density 0.75 which is same as an 18% grey card. So meter just pure white and close down 5 5/6 stops.
Or alternatively you could meter a grey card which you know is 18% reflectance so is 2 1/2 stops less than white(18% is 2 1/2 stops less than 100%). This will then get 1.0logH (3 1/3 stops) over exposure (due to the way a spot meter works) so to bring it back down to what you metered you close down by 3 1/3 stops.
And that means roughly speaking that for slide film using a reflection meter with assumed baseline of 100 speed for internal processing and K = 12.5 you can meter anything you like and close down 3 1/3 stops and it will be fairly close to original tone assuming its on the straight line of the curve which a midtone such as a grey card should be. Different film curve shapes may well affect this so I think there will be some trial and error to get it spot on.
Please try out my theory for slide film and report back. Trying both metering white and metering grey card would be good. I think colour accuracy would be better metering white because you are not dependant on grey card angle which if you didn't get it right would throw off your exposure. Back of Kodak Grey card is white so would be a good target for test of theory but could be bright clouds or some such if it exists in the subject.
Fuji Provia 100 III H&D Curve