Figure 11–5 Characteristic curves for the two components (dashed and dotted lines) of a hypothetical variable-contrast paper. Changing the ratio of blue to green light in the exposure shifts the curves horizontally relative to each other. The characteristic curve we see in the print (solid line) is the sum of the two component curves.
Figure 11–6 At low contrasts (low blue/green light ratio), the curves are spread so far apart that the toe of one curve may fall on the shoulder of the other. The result is the double-S curve shown here. This curve-splitting produces poor midtone contrast and dull prints. Different VC papers have different susceptibility to curve splitting.
Of the VC papers I’ve tested so far, Agfa Multicontrast Premium maintains the best midtone separation at low contrasts. Try this paper if you feel that your low-contrast VC prints lack a certain liveliness.
Figures 6–17, 6–18, and 6–19 on pages 72 and 73 illustrate the magnitude of the focus problem. Figures 6–18 and 6–19 show highly magnified sections of two prints. Figure 6–18 was made at the lenses’ true best focus for the print paper. Figure 6–19 was made at the best visual focus I saw using the Peak Focuser. This visual focal plane differed from the best focal plane by about 14 mm. The cause is an optical defect called longitudinal chromatic aberration (LCA, Plate 29). It’s the dominant residual aberration in most good enlarging lenses, although such lenses are adequately corrected for LCA over the red-green-blue part of the spectrum. Unfortunately, modern VC papers respond into the near ultraviolet (UV ), where the lens may not be well corrected and the human eye sees almost nothing. Even when we focus our enlargers through a deep-blue filter, we’re using a different part of the spectrum than the print paper sees. Because the print paper is reacting to light outside the normal spectral range for which the lens is corrected, the paper may see a different plane-of-best-focus than the eye does. This is a different source of focus error than the one Patrick Gainer has discovered (see Chapter 6).
F OCUSING P ROBLEMS S PECIFIC V ARIABLE -C ONTRAST P APERS
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When researching the unknown, I’ve found that the realm of Murphy’s law borders on that of Serendip. With proper application of scientific detective work, unexpected glitches can lead to new knowledge. While investigating the printing characteristics of different enlarger heads (see Chapter 6), I stumbled on a completely unexpected source of fuzziness in my prints. I discovered that there can be a huge difference between the plane of best focus seen by our eyes and the proper focal plane for VC print paper. I’ve seen as much as a 15mm focus error when making 8 x 10 prints from 35mm negatives, which results in a serious loss of print quality! My research suggests that this problem affects many (fortunately, not all) users of VC papers.
Focusing Problems Specific to Variable-Contrast Papers
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