Beyond the Zone System by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 978-1-136-09293-0
Publisher: Focal Press

Figure 7–21. Plot the speed points against their respective N-numbers. The points will probably not fall in perfect alignment. Connect them with a smoothly drawn line, averaging the errors, as shown.

You’ll probably find that these data points don’t fall into neat alignment. This isn’t as serious as it may seem to be for at least two reasons. In the first place, you can’t use speed numbers that you might read between the lines on this chart because your meter’s film speed dial won’t accommodate them. Therefore, if the curve staggers around a little, it doesn’t matter because you’ll have to use the value of the line the curve is closest to whether it actually falls on the line or not. In the second place, the irregularity of this line dramatizes what may be quite minor distortions in the shape and spacing of the characteristic curves’ toes. Drawing the EFS line so that it averages these points, rather than necessarily passing through each one, usually tends to correct those original drawing errors. Even in the worst case, the EFS error is unlikely to be greater than about ⅓ stop, and that can be discovered and adjusted on the basis of careful field tests.

You’ll also notice that this EFS chart, compared with the one you constructed in the Chapter 4 exercise, indicates less speed variation for the various N-numbers. This, of course, results from the fact that these EFS numbers are derived from the fractional-gradient IDmin values, rather than the fixed IDmin density values that we used originally.

Which values are the correct ones? You’ll get printable negatives with either set. These fractional-gradient-based speeds generally produce high-quality negatives of minimum useful density, leaving very little leeway for underexposure error. The fixed-IDmin speeds also produce negatives that yield generally excellent prints, but their safety factor varies from adequate (for short-range subjects) to excessive (for long-range subjects). In general, I think you’ll get most consistent results with the fractional-gradient set, although if you prefer not to work so close to the low limit, you can decrease the EFS values by ⅓ stop or so. That will increase the effective exposures and provide a somewhat greater safety factor without compromising image quality.

You now have working information that applies specifically to your chosen materials and includes the influence of your personal darkroom procedures. Test your chart information in the field, and keep careful records. If you find that your negatives are consistently underexposed (losing important shadow separation and detail), shift the speed numbers on your EFS chart one square or more to the right (Figure 7–22). If overexposure is a consistent problem, shift the EFS numbers to the left.

If your negatives are too flat—that is, if they produce low-contrast prints on your chosen paper grade—increase your SI number; if the print results are too contrasty, decrease the SI number. In either case, use the new SI number to reconstruct the IDmax line on your film curve graph (Figure 7–23), recalculate the SBR and N-numbers, and plot a new development chart.

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