Quote Originally Posted by mikewhi

Also, this idea of rating film at varaious ASA's and pushing and pulling processing are really terms that show a lack of the basic concepts of sensitometry. I am not putting anyone down, so please don't throw rocks at your monitor. I simply mean that for your camera\film\dev there is one and only one correct ASA speed. By the definitions of ASA (American Standards Assoc), the ASA speed is that which produces .1 above film base+fog. There is only one ASA speed that can do that, and your mission, if you choose to accept is to find that speed. If caught, Ansel Adams will disavow any knowledge of your actions.

Mmh... that's why I call it an EI (exposure index). Make it a PEI -personal exposure index-, if you wish.

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.

Saludos, Santiago