Why is Zone System EI often about half rated ISO/ASA?

Plotting the x and y axis of the H&D curve as logarithm allows one to understand and manipulate the data with addition and subtraction.
Each stop is 0.3 log, so you can use simple math to find out distances on each axis in stops.

Otherwise one would encounter problems like determining the number of stops between,say 30,456 and 417, etc…

But not without some confusion, where negative numbers on the axes can have one scratching their head over negative values of light, when indeed it is merely a representation of light values less than one.

ic-racer said:

Plotting the x and y axis of the H&D curve as logarithm allows one to understand and manipulate the data with addition and subtraction.
Each stop is 0.3 log, so you can use simple math to find out distances on each axis in stops.

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As an electrical engineer I'm very familiar with logarithms. I just wasn't familiar with the notation used here.

Understandable, H&D curves were created years before any Si standard nomenclature.

ic-racer said:

Understandable, H&D curves were created years before any Si standard nomenclature.

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Actually the SI Standard Nomenclature was already established. The H&D curves were developed in the absence of SI Standard Nomenclature.

SodaAnt said:

That makes sense. I guess you just have to be familiar with the notation used. What does the "(3 1/3 stops)" part mean?

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It refers to the difference in stops between the metered exposure and the measured speed point.

Sirius Glass said:

Actually the SI Standard Nomenclature was already established. The H&D curves were developed in the absence of SI Standard Nomenclature.

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Ok, I was thinking 1960 or, so indeed wiki shows Si originating 1875, about the same time the H&D curve was developed.

Anyway my initial comment referred to my curiosity over the unit “E” on the X axis, vs “H”

Seems related to them using a light integrating sensitometer rather than a step wedge.

This is the nomenclature from a Jack Holm paper.

The paper you shared is a great snapshot of where sensitometry of black and white negative photography was in 1943 before the Zone System.

Some key numbers are easier to remember in arithmetic notation and some are easier to remember in Log10 for example “32” (the range of exposure Jones wants you to consider when looking for the 0.3 gradient to use for speed determination) is 1.5 Log10.



And you know that when you change an f/stop you cut the light by half or increase it by double. That works out to be 0.3 Log10.



The Zone System asks you to determine your speed at the point where you have stopped down by four stops (from Zone V to Zone I).

That makes the Zone System shift from speed determination point… 4 times 0.3 or 1.20 Log10.

Four stops is 1.2 but the ASA method aims 1.0 Log10 from speed point to meter point. That’s the number that is easily remembered arithmetically as “10”.

That’s where the 3 1/3 stops comes from.