This:
http://www.largeformatphotography.info/articles/conrad-meter-cal.pdf is the source that Hannemyr references for his meter calibration information.
Lee
I just wrote a lengthy post about Conrad's article and lost it before I could post it. So, here's another try without as much detail.
Conrad is mistaken on a few topics. First, his assumption for the checkerboard having the white squares equal 90% reflectance. He then calculates an average reflectance of 18 - 19%. He might just be using this as an example, but I've seen people attempting to shoehorn figures in order to achieve a predetermined result before. For theoretical exposure determination the highlight is assumed to be 100% reflectance. The statistically average scene luminance range is 2.20. The distribution of the highlight is about 0.95 above the mean log luminance or midpoint and the shadow is 1.25 below. Add a stop flare to the shadow and that brings the shadow up to 0.95 below the mean and you get 0.95 above the mean and 0.95 below. That's just under 3 1/3 stops and 3 1/3 stops is the difference between the midpoint exposure and the speed point (coincidence? Nope).
He also claims that K is only a metric conversion factor. That's incorrect. It's a light loss constant much like q is in the major exposure equation - q*L*T / A^2 + Hf = H.
He is also mistaken in claiming Hg or midpoint exposure as 11.38Hm. It's 10Hm. The midpoint uses a constant "P" which is equal to 8 lux. The standard says that it is more accurately 8.2, and I've found it works best with my calculations at 8.11 but 8 is fine for most situations. Consider the Sunny 16 rule and the basic exposure equation H = E x t. P would equal E and t would be the 1/ISO of the Sunny 16 rule. For a 125 speed film, that would equal 0.064 lux seconds for Hg. Speed point exposure or Hm would be 0.0064 lux seconds for a 125 speed film or 10x (1.0 logs) less. All this is described in Connelly's paper,
Calibration Levels of Films and Exposure Devices, which I referenced in an earlier post.
He is on the right track with his explanation as to why Zone System practitioners generally obtain effective film speeds 1/2 that of the ISO rating. He doesn't go into much detain and never mentions the important influence of flare.
His statement, "ANSI/ISO 6-1993 specifies a higher contrast than usually is desired for practical photographic use," doesn't consider how the standards contrast parameters infers the Delta-X Criterion, which is a mathematical approximation of the fractional gradient method. According to Delta-X, film speeds change little with changes in processing. In fact, when rounded to the nearest third stop, a given film/developer combination will produce a single film speed for a development range between -1 to +2.
Conrad's problem is that he is mostly using the standards as his source and not the papers from which the standards are derived. The purpose of the standards is to describe how to do something. How to determine film speed or how to calibrate an exposure meter. They don't explain the reason behind it. Generally, if there are major changes proposed, a paper with all the reasoning and theory will be published around the time the committee is considering a new standard. Here again are three of the key papers explaining exposure and exposure meters.
Connelly, D, Calibration Levels of Films and Exposure Devices, Journal of Photographic Science, Vol 16, 1968.
Nelson, C.N., Safety Factors in Camera Exposures, Photographic Science and Engineering, vol. 4, no. 1, Jan-Feb 1960.
Stimson, Allen, An Interpretation of Current Exposure Meter Technology, Photographic Science and Engineering, Vol. 6, No. 1, Jan-Feb 1962.