Originally Posted by SchwinnParamount
I'm not sure I am following that. Are you saying that 100 asa b&w print film has a different speed than 100 asa transparency? or that transparency is best when it is under-exposed. If that is the case, why isn't a 100 asa transparency rated 80 asa? Or have I completely missed the point?... a very likely possibility
The speeds of the different materials are determined at different points of their curves. While B&W film film speed is determined at a point in its shadows, transparency film is determined in the middle of its curve. The reason why they have the same ISO, is because of their speed equations are different. It's the beauty of photography that they were able to tie all of this together and make it work most of the time.

B&W - 0.8 / Hm - Hm = exposure at a density of 0.10 over fb+f
Trans - 10 / Hm - Hm = exposure at film mean - mid point

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An good equation to illustrate Sunny 16 is the exposure meter balanced calibration equation.

A^2 / T = (B * S) / K

Where
A = f/stop
T = Shutter speed
B = luminance in footlamberts - average is 297
S = film speed
K = constant = 1.16

Plugging in Sunny 16 for a 125 speed film:

16^2 / 1/125 = (297 * 125) / 1.16

1/125 and 125 cancel out leaving

256 = 297 / 1.16

297/1.16 = 256

256 = 256