You wrote about the coincidence between Sunny 16 and standards in this thread...http://www.apug.org/forums/viewpost.php?p=295310
It's worth revisiting the idea that Jones figured out how bright the sun is (to oversimplify) the same year the standards came to be.
I couldn't find the brightness of average daylight conditions in your proofs. Is it there?
The proof for illuminance comes from the incident meter calibration equation.
A^2 / t = (I * S) / C
Using Sunny 16 and solving for I, it becomes I = 16^2 * C
According to the exposure meter standards, C = 30
256 * 30 = 7680 footcandles
The average reflectance between the illuminance of the incident meter and the luminance of the reflection meter is:
(297 * pi) / 7680 = .12
You can also calculate the average reflectance using the constants:
(1.16 * pi) / 30 = .12
According to the ANSI photographic exposure guide, "Daylight reaches a maximum illuminance of approximately 11,000 footcandles at a solar altitude of 90 degrees." The solar angle used for the measure is approximately 40 degrees. According to the standard, this is approximately 2/3 of a stop less light than the maximum.
I've done a little calculating and 11,000 footcandles at 90 degrees will yield 8426 footcandles at 40 degrees and 7778 footcandles at 45 degrees. For the illuminance to equal the incident exposure meter value of 7680 at 40 degrees, the maximum illuminance would have to be around 10,200 footcandles. I think that's only a tenth of a stop difference.
The way time and speed cancel each other out and aperture becomes a constant, 16 that is easy to square.
Eerily coincidental. Or as you say not a coincidence at all.
Did Sunny 16 work the same before and after the safety factor was removed?
You need to think about each part on it's own. The scene luminance range and illuminance doesn't change, the exposure meter doesn't change, it's the placement of the exposure on the film curve that changes based on the speed point / exposure meter ratio.
Let me think a minute.
It was ASA (old standard with safety factor) 50 so f/16 at 1/50th... Now it's ISO (current standard) 100 so f/16 at 1/100th...
OK the exposure did change.
Bill, I like to come at the problem from many different directions, and if the answer is still the same, there's a good possiblity it's correct.
Everything in photography is interconnected. A piece of information might seem to work on its own, but plug it into the bigger picture and it doesn't quite fit. Like with the reflected exposure meter reading 18%. That sounds right until you compare it with the incident meter's illuminance. A normal negative should have a density range of 1.25 to fit on a grade 2 paper sound good until you compare that to the paper LER range of 1.05 for a grade 2 paper. On the other hand, I'm happy to say the Sunny 16 seems to be able to hold up under scrutiny.
That's why I really like the constants equation. It shows the interconnection between the vaious elements involved in the photogaphic exposure by combining the constants from the exposure meter, K, the camera, q, and the photogaphic exposure, P, into an equation.
K * q = P or P / q = K or whatever variable you're solving for. K is in cd/m^2 or nits.
12.48 * .65 = 8.11