Response to Film Manufacture Recommended Times ,Huh?

Jeff and others have given very good explanations, but I'll add a little bit also.

I can't speak for other manufacturers, but at ILFORD we run development tests to give a Gbar of 0.62 (Gbar is a slope measurement, similar to but different from Contrast Index. The methods are different enough that there is no conversion method). This same contrast ratio is used for all developers, and the time that gives this contrast is what is published, rounded to the nearest 15 seconds. The testing used for developing times is not connected with the ISO testing for film speed. ISO measurements use exposures made by calibrated machines, and processed in very exacting conditions. These exposures and processing conditions bear little resemblance to most photographic situations; it is simply a standard that is easily replicated in the laboratory, regardless of where you are. ISO speeds of different brands of film are directly comparable, but only as far as the ISO standard goes. For your particular shooting style (including subjects, lighting conditions, and equipment), you may see variations between two different films having the same ISO speed rating.

On developing, there are a large number of variables. Different people will not only have different techniques, but also different ideas of what constitutes a "perfect" negative. Added to uncalibrated thermometers, graduates, etc., this can lead to vastly different optimal times. Quite often, people will comment to me about our developing times. Most of these people say that our times are "right own", and say they are better than the numbers other companies publish. But other customers will complain, saying that our times are too short or too long. I would guess that people vary as much as -30% to +100% from our published times.

So the bottom line is that published numbers are good to use for starting points for your testing. The good news is that as long as you stick with one manufacture's numbers, the difference will probably remain roughly constant.

David Carper ILFORD Technical Service

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