Approaching the Curve

Now comes the first really subjective part of my processing. It’s obvious that much of the information I wish to display resides in a small area of the histogram near the faint end. Using the Curves function, I perform the equivalent of a hyperbolic arc-sine adjustment based on the image data. I first open the Histogram palette and determine the values that are slightly darker and lighter than the bulk of the histogram data. I then apply a steep linear slope to the curve between these levels. I use a gentler slope for the curve above and below this mountain of data, which prevents the faintest data from being clipped to black and the brightest highlights from clipping to white.

Using the Curves palette, you can now stretch the image, though carefully monitoring of the white and black points is still required to avoid clipping the ends of the histogram. Often more than one application of the Curves function is necessary to achieve satisfactory results. This particular image required the two curves presented above, plus one additional, though less aggressive, application of the function to establish the proper contrast throughout the image. (Click for larger image.)

Chuck Vaughn

While this adjustment improves the image greatly, my subject still appears faint, and the background is still too bright. I need to apply at least one additional curve to increase the contrast. I open the Curves palette again and place additional points to hold the linear part of my curve straight while simultaneously keeping the shadows and highlights from clipping. Exactly how I adjust this curve depends on what it’s doing to the image on the screen — I try to achieve good contrast in the object, a dark gray sky, and highlights that are not burned out. I may try several curves until I find one to my liking.

One rule I apply to all my images during processing is that objects must fade into the background sky rather than have razor-sharp cutoffs. It’s okay for regions of the subject to be faint. Typically I leave the sky background in the range of 20 to 30 out of the 255 levels displayed in the Histogram window. Another, weaker Curves adjustment might be necessary to achieve the desired effect.