Mmh... that's why I call it an EI (exposure index). Make it a PEI -personal exposure index-, if you wish.
Originally Posted by mikewhi
There are many ASA speeds possible that produce .1 above base fog; just make development time variable. This will push up the characteristic film curve, maximally in the highlights, less in the medium tones, and minimally (but noticeably) in the shadows. In other word, development time affects negative contrast and speed.
There are many other variables, other than development time, than affect film speed. As an example, consider the color temperature of the light used in the exposure (daylight, tungsten or ?), the agitation method and time (continuous, intermitent every 30 secs, every 60 secs, stand development?) , and which developer (rodinal, microphen, others?) was used to process the film
ASA just standarizes those variables, using daylight as light temperature, using a standard agitantion method, using a specifically determined developer, and developing as long as it takes to reach a contrast index of 0.62.
After that, and reaching an tentative speed, it multiplies it by 0.8, to help prevent people underexposing by claiming film is 20% slower than it really is.
My point is that ASA speed is a (very?) good approximation of the film speed, but unless you develop it exactly as specified in the standard, there are no guarantees that YOUR speed, with your developer, agitation method, contrast preference or whatever, will be the same.