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Date

LuX-Second Conversion

Eino Lilback

13:38 23 Sep 02

Re: LuX-Second Conversion

Phil Davis

18:18 23 Sep 02

Eino --

I don't know of any way to make an accurate conversion. Since we don't have any way to duplicate the ISO Standard's test procedure, we work on the assumption that averaging the response of several "standard" films in general-purpose developers will provide a reasonable guide for assigning relative speeds. We've identified several film/dev combinations that appear to be speed rated honestly by their manufacturers and we use the discovered average speed point of a curve whose average gradient is approximately 0.62 as representing the "ISO" speed point. The exposure condition that produces that speed point becomes the basis for subsequent tests. We go to some lengths to standardize these test conditions among BTZS users so that we can exchange test data with some success.

This is admittedly not "official" but it has proven to be a workable standard for home use.

We use the enlarger for test exposures because it's convenient, it provides light of sufficient uniformity and intensity to be satisfactory, and the usual electronic timers provide accuruate and repeatable (at least repeatable) exposure times.

We don't use standard daylight illumination because it isn't easy to provide under home darkroom conditions and we rationalize that decision by recognizing that many (probably most) photographs are not taken in "standard" daylight. We don't use an approved exposure time because most enlargers don't provide bright enough light to permit times of much less than 1/2 second; and we don't use a special test developer either — but neither does ANSI/SO anymore.

In short, the whole point of this approach is to mimic the "standard" conditions as closely as is practical in the home darkroom, and to establish an objective standard for determination of _relative_ film speeds. On the basis of these test conditons we can't challenge the accuracy of the manufacturers' assigned speeds, but we can determine whether those ratings are useful for our purposes.