This article appeared in the March/April 2002 issue of PT. To purchase this issue and receive this and other valuable articles in this issue, CLICK HERE: Ship within the U.S. | Ship outside the U.S.

Photographers often wonder how electronic imaging works, and more importantly, how to get the best results given the wide range of unfamiliar terminology and drop-down menus. One important photographic concept is the film characteristic curve. In this article, we’ll show that the characteristic curve applies not only to photographic images, but also to digital imaging. If you understand the characteristic curve and the effect that changing contrast has on an image, you can apply this to electronic imaging. We’ll review the film characteristic curve, then demonstrate that the same concept can be used (with only a slight name change) in Photoshop and monitor calibration programs. We’ll show you how to draw a characteristic curve for a digital imaging device. While “transferable skills” may be a buzzword, it’s certainly applicable as we make the transition from silver halide to CCD.

The photographic curve
Let’s start with a quick recap of the film curve. The photographic characteristic curve is used to show how film behaves when used in a camera. The curve is published in technical data sheets, or you can draw it yourself if you have a densitometer. The curve is a plot of log exposure against density, i.e. a plot that shows how much developed density we get on the film in response to the light falling on the film at that point. Kodak T-Max is a general-purpose, black-and-white material capable of reproducing a wide and continuous range of gray tones. Kodalith Ortho is a very high contrast material used in copying line reproductions that are black or white, with very little tonal information. The curve for Kodalith is very steep. My point here is that the appearance of the image depends on the contrast of the chosen film. If you choose extremely low-contrast material, you can expect an image that may be “washed out.” Likewise, a high-contrast film may produce an equally unpleasant stark image. Only a medium-contrast film with a medium slope will produce a pleasing range of continuous tones.

It’s difficult to describe what “low” and “high” contrast mean, so there’s a mathematical number we can use. The number is called “gamma,” and is a measure of the slope of the characteristic curve. Its precise definition is a measure of the slope or gradient of the straight line portion of the characteristic curve. A general-purpose film has a gentle slope with gamma of perhaps 0.8, while a high-contrast film has a steeper curve with a gamma of about 2.0 or more. To calculate gamma, use the following equation: Gamma = DD / D log exposure, where DD refers to a difference in film density, and D log exposure refers to the corresponding difference in log exposure. When we move to digital imaging, there are two things we must remember. First, just as the choice of film affects the look of images, the choice of gamma setting for digital imaging affects the reproduction of digital images. Second, gamma is just a numerical way of specifying the slope or contrast. A higher gamma means higher contrast.

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