Excepting hand-held, available-light aficionados, under-exposure is something we tend to avoid like the plague. But overexposure? No problem. All black-and-white camera films handle overexposure with ease. Or do they? It depends on the shape of a film’s characteristic curve.
Many photographers, especially fine-art photographers, routinely overexpose film relative to its ISO speed rating. Typically, this is done to capture additional shadow detail in the negative, affording the opportunity to include it in the final print, or not, once it is time to make that print. Two films may furnish rather similar tone reproduction when both are exposed “normally,” based on their ISO speeds. But overexpose both, and even with careful attention to processing, tonal differences can become profound.
Our March/April 2005 column discussed differences in negative appearance that result from two different film-curve shapes when both are “normally” exposed at the standard ISO speed. Here we will overexpose both films by two stops.
Figure 1 illustrates the pair of curves, one (pink) with a rolling shoulder and the other (blue) with a slight lower scale sag. The double-headed arrow indicates placement of an average scene on the Log E axis with “normal” exposure. Processing is adjusted to match the shadow and highlight densities of the two at values that exactly fit a grade 2 paper. Figure 2 demonstrates what happens if both films are overexposed two stops. Note the double-headed arrow indicating scene placement is shifted 0.6 Log E (two stops) to the right. Processing has been readjusted to again insure the respective differences between shadow and highlight densities fit a grade 2 paper. This required almost no change to the pink curve film (overexposing does not automatically mean “pull process”), but the blue one did need a substantially shorter development time to maintain a grade 2 density range. This accentuates the effect of the sag.
All the modest appearance differences previously discussed for normal exposures are now greatly exaggerated. This is illustrated with a pair of prints. Figure 3, featuring a film with a rolling shoulder (pink curve) has more open shadows, lighter midtones and more compressed highlights. Figure 4, the more linear (blue) curve, does the opposite: Shadows compress, midtones are darker and highlight tones expanded. Recognize the whitest white and blackest black in the two prints are identical. They differ only in their film curve shape, the path between those extremes.
Which curve shape is better? Not for us to say. It depends on the subject matter and the photographer’s intent. The point is that film characteristic curves are not the exclusive province of techies. A glance at film curve shape, as published by the film manufacturer, goes a long way in helping any of us select the film best suited to a particular picture opportunity.