I ran across these examples of speed methods for various film types (and one paper). This is the only time I've seen diagrams of non-pictorial films included. It's a bit dated, but I think it illustrates how the method of determining the speed is dictated by it's intended use.
Last edited by Stephen Benskin; 01-21-2012 at 01:08 AM. Click to view previous post history.
is the graph showing photographic paper typical for all photo papers ?
i have tested 20+ types/brands/grades of photopaper and they vary
greatly in speed ... from asa 25 or 50 to less than 1
( asa relative to film speed, not the index numbers the companies give )
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Stephen, thought I would ask you this question even though it is not necessarily the right topic, although definitely related. Why do so many people target a value for zone III when they test film for reciprocity adjustments? Why not aim for a repeatable zone I value, since .1 over base+fog is generally how most zone system users test for their working EI? Wouldn't it make more sense to target a constant EI for reciprocity corrections rather than zone III?
I'm guessing that choosing Zone III for reciprocity failure tests avoids the situation where the loss of speed leaves you too little information at the normal 0.1 speed point.
I don't understand what you mean.
Stephen, yes I've seen this method used in several high profile tests. For example, Howard Bond's well known tests have been quoted by many (even myself). He calibrated to zone III. I've seen zone III referred to in other writings on reciprocity as well. In fact, offhand I can't recall seeing any articles or books in which zone I was used as a reference point for reciprocity corrections.
I'd have to read those articles. Reciprocity only effect areas of low exposure (except for high intensity reciprocity, but that works differently). The principle is the same as latent image keeping as explained in the Gurney-Mott hypothesis. I don't remember much about chemistry, but here's an excerpt from Photographic Materials and Processes on reciprocity:
"When exposures are made at low light levels, the efficiency with which the sub-latent images are formed on the crystal is low because of the tendency of the silver atoms to give up an electron and return to the ionic state. Further exposure allows a greater number of silver atoms to accumulate around those few sub-images that survive, but these sub-images are relatively stable."
It seems to me that since reciprocity has a greater influence with lower exposure levels, and since the purpose is to define the film speed at lower exposure levels over a longer period of time, then the speed point would be a logical point of measurement.
Again, I'd have to read the other people's reasoning in order to comment on them. But I frequently seem to be at odds with popular wisdom.