RobC, you forced me to go back to your old post #215 and read what you wrote. I did not find you gave me the answers to my questions.
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
These are 4 arbitrary points of known exposure on the curve, aren't they?
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
You really are obscure here. You mean you meter whatever subject, you take note of the exposure, you place it on the LogH axis, and you then shift 3.3 EV to the right (meaning: more exposure). What you get is an exposure which is 3.3 EV above what you read. You still have no idea how wrong is this exposure because you did not evaluate the reflectivity of the subject. The only case this works is if you meter a very dark cat that you know is 3.3 EV below middle grey (which is what your meter gives you). Then you open 3.3 EV and you get the right tone for the cat. If you meter a grey card, you open 0.0 EV because the metering you get is correct for the grey card.
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.
You mean we meter something which we want full white, and actually is full white. And it's the opposite. When you meter a white object, your reflected spot (or average) meter will give you an
underxposed picture. You will be vastly underexposed into the shadows.
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.
If you meter a white object you already are some 2.5 EV underexposed. If now you close another 5.5 EV and more you are just challenging the D
max capabilities of your slide process.
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.
No way on earth. If you meter a grey card of known 18% reflectance you get - 2.5 EV stops less than white - but, the card being grey, you get spot-on exposure, you don't get overexposure, due to the way a reflected light meter works. If now you close 3.3 EV your Kodak card becomes as dark as a Nespresso coffee.
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.
A reflected light meter doesn't have a "baseline speed". How could it have it? It measures light, and then, thanks to a certain set of mathematical parameters, it will give you the correct exposure
for its target grey for any film speed. The light meter will produce always the same density on film (if fed with the correct ISO). That density will be the correct one if your subject is 18% grey. Else you have to know that your subject is white and open 2.5 EV so that it becomes white.
You can't meter anything and assume it will come out of its original tone just because you closed 3.3 stops!
You first evaluate how your subject is relative to middle grey. You then take a spot meter knowing that it will give you a reading that will make the subject middle grey. If your subject is skin tone, let's say 1 EV above middle grey, then you open 1 EV above the metered exposure. But if your subject is darker than middle grey, let's say is half the reflectivity, then you close 1 EV below the metered exposure.
All this reasoning presumes not only that you can evaluate the reflectivity of this subject, which is 1 EV below middle grey, but also the you know where middle grey is, otherwise you cannot know where your dark subject is relative to middle grey.
Buy some slide film and do test your system, and "my" system, and come back to report your findings. Do it with cheap film!
My light meter doesn't know how the film curve is. He doesn't even know if I use negative or slide film. I must know it. That's why I must study the characteristic curve of the film. So that I can know that I begin losing texture on white at 2.3, or 2.5, or 2.7, or 3.0 EV above target grey.
Which means that I have to be able to locate target grey on the chart, to begin with, and I have to be able to locate it in reality, because every single measure I do with my spot reflected light meter begs the question: "how brighter or darker is the tone that I measured, relative to middle grey?"!
If I don't ask myself this question, I wasted the money I spent on the light meter, and I am WAY better off using the internal averaging TTL meter, because that 90% of the times will have a scene in front of it that will be, on average, of middle grey.
Oh, that author was me.
