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It looks like three of the methods (0.3xGbar, deltaX, and W) are about equally good. The 0.1 method is worse, both in terms of a bias and greater variability.
A quick little history.
To find the speed of film researchers Jones, et al 'discovered' that if the shadows fall on a part of the toe curve that is tangent to a line one-third of the slope, that was the minimum exposure for an excellent print. They made multiple prints and showed them to a panel of observers.
When they did this, there were no personal computers and this 1/3 slope or 1/3 G or 'fractional gradiant' was not so easy to find outside of using pencil rulers and paper graph.
So...three approximations of this 1/3 G (fractional gradient point) were developed that could be found on the toe curve without as much trouble or without a computer:
0.1
Delta-X
W-speed
The three methods were compared and found to be roughly similar in the approximation of 1/3 G speed point.
It turns out 0.1 was eventually chosen as the method to use by modifying it slightly to specify a certain slope the film needs to have (so called 'ASA Triangle') before taking the measurement. This became ASA and was nearly copied directly for the ISO standard and also embraced by many Zone system users.
This whole history of film curve analysis would have probably played out differently had computers been available.
So, as it stands ISO is firmly established, but it is fun to look back at how it came to be.
Realize that in all this research the GOLD STANDARD is "print judgement." Yes, objective viewing of the prints by a panel of observers TRUMPS ALL THE MATH!
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Glad you said that, I had the same thought.It looks like three of the methods (0.3xGbar, deltaX, and W) are about equally good. The 0.1 method is worse, both in terms of a bias and greater variability.
When that paper was published they were ONLY looking at the 0.1 point of the film without specifying the contrast or development time for that film. If one were to check 0.1 point with a specific contrast development for the film, then 0.1 is very good. That is the ASA definition that got turned into the ISO definition.It looks like three of the methods (0.3xGbar, deltaX, and W) are about equally good. The 0.1 method is worse, both in terms of a bias and greater variability.
One thing I have never quite been able to figure out is what is the relationship between H in the figure and an ordinary light meter reading of a blank image.When that paper was published they were ONLY looking at the 0.1 point of the film without specifying the contrast or development time for that film. If one were to check 0.1 point with a specific contrast development for the film, then 0.1 is very good. That is the ASA definition that got turned into the ISO definition.
However to make this determination you need a family of film curves to select the correct one to measure the 0.1 point. An advantage of Delta-x and W-speed is that you don't need to make the family of film curves (ie different development times).
Also see ( https://www.photrio.com/forum/threads/speed-point-0-1-over-base-plus-fog-or-1-3-of-gamma.183061/ )
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One thing I have never quite been able to figure out is what is the relationship between H in the figure and an ordinary light meter reading of a blank image.
A quick little history.
To find the speed of film researchers Jones, et al 'discovered' that if the shadows fall on a part of the toe curve that is tangent to a line one-third of the slope, that was the minimum exposure for an excellent print. They made multiple prints and showed them to a panel of observers.
When they did this, there were no personal computers and this 1/3 slope or 1/3 G or 'fractional gradiant' was not so easy to find outside of using pencil rulers and paper graph.
So...three approximations of this 1/3 G (fractional gradient point) were developed that could be found on the toe curve without as much trouble or without a computer:
0.1
Delta-X
W-speed
The three methods were compared and found to be roughly similar in the approximation of 1/3 G speed point.
It turns out 0.1 was eventually chosen as the method to use by modifying it slightly to specify a certain slope the film needs to have (so called 'ASA Triangle') before taking the measurement. This became ASA and was nearly copied directly for the ISO standard and also embraced by many Zone system users.
My understanding is that H is the exposure in lux seconds, which implies that H is not a floating number. (Reference: https://en.wikipedia.org/wiki/Film_speed) However, I don't know how to convert a light meter reading to lux seconds.There isn't one. The H axis is a floating logarithmic scale indicating relative exposure. What's it relative to? Well, whatever amount is needed to expose the material adequately for making the graph. The axis may indicate light intensity, exposure time or a combination of the two.
My understanding is that H is the exposure in lux seconds, which implies that H is not a floating number. (Reference: https://en.wikipedia.org/wiki/Film_speed) However, I don't know how to convert a light meter reading to lux seconds.
My understanding is that H is the exposure in lux seconds, which implies that H is not a floating number. (Reference: https://en.wikipedia.org/wiki/Film_speed) However, I don't know how to convert a light meter reading to lux seconds.
So, to be sure I understand what you are saying, does this mean thatMetered exposure is 8/ISO. Black and white speed point is 10x below that. 8/125 = 0.064. Speed point for125 = 0.80/0.0064.
I think there is an error in what you wrote. Watt seconds doesn't make sense on its own. There must be a unit of area somewhere, such as meter^2.The X-axis is really in watt-seconds (WS). Lux-Seconds is a photometric unit related to the response of the human eye. Imagine characterizing an IR film in LxS - in the infrared the LxS reading is always 0.0 as the eye can't see IR. But, as LxS are easier to measure, and the two are related, for a given light source, by a constant multiplier LxS is used as a unit of measure. This multiplier, when applied to the logarithmic H/X axis just shifts things to the right or left; the shift is of no consequence as the H values are all relative.
But how many WS is plotted on the H axis is another matter. You can plot a characteristic curve from the exposure of a strip of photographic paper to a test strip f-stop/log10 sequence starting at the white point (almost but not quite entirely white) and proceeding in f-stop/log10 intervals. When the H-axis is plotted the tick marks are really each test strip patch. Yes, the paper is exposed in 'lux-seconds', and they are plotted in a logarithmic scale, but where the zero point is depends on the material being exposed and how bright the light source is and it's spectra. These are elements that are irrelevant to the curve where only the shape matters.
If you want to calibrate an HD curve to your equipment pick a density in the middle of the curve and determine the exposure that results in that density. Then you can label the X-axis in absolute units for your setup.
Some meters, Weston Master series?, have a chart on the back that relates meter readings to lux.
I think there is an error in what you wrote. Watt seconds doesn't make sense on its own. There must be a unit of area somewhere, such as meter^2.
I found more information on the relationship between light meter reading and the amount of light per unit area on the subject. The reference is a paper titled Exposure Metering Relating Subject Lighting to Film Exposure, written by Jeff Conrad. Here's an equation from the paper.
View attachment 297813
Here's a definition from the paper
View attachment 297814
The author also gives the following definitions for the quantities in the paper. "where EV is the exposure value, S is the arithmetic ISO speed, and K is the meter calibration constant." And for Nikon or Canon equipment K=12.5
One thing that is missing from the equation is the reflectance of the subject. What if we assume the2$ average reflectance equals 12.5? That might (possibly) relate to the constant, K, in the equation.
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