Besides, there is something I don't understand.
How can a calibration Luminance be directly correlated to a certain reflectivity, as the table does.
There certainly are a lot of other variables that must be populated before you arrive, in a calibration process, from a certain Luminance used in calibration, to a certain density. A relation must be defined somewhere!
That's the point. It isn't calibrated to reflectance. The value is an equivalent based on the value of the calibrated Luminance and what is considered the value for illuminance. Now if the illuminance is different, the Luminance value will have a different equivalent reflectance. That's why there is no definitive reflectance. The Minolta guy says 18% because it can be the equivalent of 18% depending.
Diapositivo and Benskin. I think its a love match.
I'll set this aside for the moment. You know my point is that there must be an implicit reflectance hardwired in any reflected light meter, but we set this aside for the moment, I have a more urgent question for you, and that regards speed equations.
For a speed equation of Hg = 10 / ISO, which is the speed equation for slide film after the modification, and for a 100 ISO slide film, Hg is 0.1 and the LogHg is -1.0. That's it, one must not add, or subtract, any value to this LogH because it has already be compensated by adopting a different speed equation! The different equation gives a value that can be eaten straight from the oven.
For a speed equation of Hg = 8 / ISO, which still is the speed equation for negative film if I got this thing right, and for 100 ISO, Hg is 0.08 and LogHg = -1.1.
What rings a bell in my ear is that I would expect, at ISO parity, less exposure for slide film, not more. Because slide film was considered, with the new equation, to be "more sensitive" than with the old equation.
If LogHg for slide film is -1.0, I would expect it to be -0.9 for negative film.
That is also why, probably, initially I inverted the two equations, thinking that 8 / ISO was good for slide film and 10 / ISO was good for negative film.
B&W negative is 0.80 / Hm where Hm is at 0.10 density about Fb+f when the contrast parameters of the standard are followed. Hg is 10X to the right of B&W Hm.
Mark, I don't think there is a standard for contrast.
So now I'm confused Stephen.B&W negative is 0.80 / Hm where Hm is at 0.10 density about Fb+f when the contrast parameters of the standard are followed. Hg is 10X to the right of B&W Hm.
So now I'm confused Stephen.
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.
So now I'm confused Stephen.
Well, yes. But he doesn't tell us that, Mr. Minolta. Mr. Minolta gives us that exposure for slides and negatives. And it's 1/3 EV less exposure than what Hg is with the speed equation.
Mr. Minolta explicitly tells us that light meter gives an exposure as HMinolta = -1.0.
Mr. Minolta explicitly tells us that he uses a K = 14. He doesn't tell us he does it because it matches recommended exposure for slide film, but I guess he does it for that reason. After all, K = 14 is outside of the ISO recommended range for values of K!
The source I referred to says that a calibration for K = 14 yields an exposure that is 1/3 EV less than for K = 10.64.
What I construct from all that is that:
Standards require K = 10.64 but then, they also require a 1/3 less EV than nominal speed for slide film!
Minolta uses K = 14 and that results in an exposure which is the desired one for slides.
That also corresponds to a calibration to a 18% grey.
Sekonic meters are calibrated for K = 12.5 which is 1/6 EV above Minolta exposure, and half way between Minolta and standard ISO. A compromise between negative and slide film.
So Minolta light meters are thought for slide film, Sekonic are a compromise between slide and negative (or slide and ISO) and that is why there is the little discrepancy.
The final answer is: Light meters with K = 14 are calibrated to a 18% grey and are spot on for slide film. Light meters with K = 12.5 are calibrated to a 15.7% grey, expose 1/6 EV more than Minolta/Pentax and are halfway between calibration for slide and calibration for negative as recommended by ISO standards.
ISO standard would require a K = 10.64 which would correspond to a 13.5% grey and would result in 1/3 EV more exposure than Minolta/Pentax, if we trust http://dpanswers.com/content/tech_kfactor.php
This is my understanding so far.
PS In fact, Mr. Minolta, in the manual of the Spotmeter, makes explicit reference to the dynamic range of slide film. He doesn't say it explicitly, but it is a device which is thought with the slide user in mind.
I thought that the ISO standard at least for B&W had a norm for contrast where Δ log-H is fixed at 1.30 and Δ D is fixed at 0.80.The speed standard's contrast parameters are about the correlation of the fixed density speed point with the Delta-X Criterion. It's not about processing to a subject luminance range.
I thought that the ISO standard at least for B&W had a norm for contrast where Δ log-H is fixed at 1.30 and Δ D is fixed at 0.80.
The attached pdf was found via google here on APUG but author is unknown and I can't find the link back to the thread.
A higher K number gives more exposure. So using a K14 meter you would give your film more exposure than if you use a K12.5 meter assuming you're using the same camera and lens.
That was actually the point I was trying to make back there in response to wiltw:For a 7 1/3 stop scene luminance range, the average gradient is around 0.58. The ISO speed contrast parameters is around 0.61 depending on the toe characteristics. There is no mention that it should be used as normal development in real world shooting conditions.
wiltw's statement is false. One is not inherently more contrasty than the other.When film is exposed, it is inherently more contasty than Digital...
I kinda thought so.Oh, that author was me.
You are absolutely right. So in this case the Minolta lightmeter would give 1/6 EV above the Sekonic, and 1/3 EV above the 10.76 standard (old Weston, maybe).
So it's not the Minolta which is "skewed" toward slide film use, it's the K=10.76, if we take as "skewed toward slide use" a light meter that keeps itself on a "slightly darker middle grey".
I find difficulties in finding the K calibration constant value in the various instruction manuals around. I found a manual for my Gossen Spot-Master (which I bought as a backup for my Minolta Spotmeter and never used, there's not even a battery in it) and the manual says reading an 18% card gives a reading of incident light. It makes no reference to a K calibration constant but constantly refers to 18% grey, not to "mid tone" or such other vague indication.
I suspect that since many years all light meters are calibrated to give a reading of incident light when measuring light reflected over an 18% reflectance subject. That would mean all lightmeters are calibrated to K=14.
The other values of K (12.5, 10.64) are probably more related to older conventions rather than to a certain producer.
Tomorrow I will see if it works and compare its response with the Minolta Spotmeter.
If you have a meter calibrated to a K=10.76 and then using cd/ft^2 instead of cd/m^2 as the luminance value then you have a K=1. This is the reason why Adams claimed that the K factor is artificial and there should be no K factor.
I have, have you?Its been over a week. Haven't you made an expsoure yet?
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