I had also hoped to address the specific software packages you can use for photo editing in this column, but based on my appointment schedule for the next month, it looks like many of these products will be undergoing their annual face-lifts soon. So I'm going to wait until early- to mid-September to write about your software options and just concentrate here on building a foundation.

Indecent exposure

Edict: Never use brightness and contrast adjustments.

The whole idea behind these two tools makes me cringe. Why? Well, right now I'm sitting in a freezing office, while some others around me are sweating, and still others feel just peachy. One temperature sensor determines the temperature for the entire floor, and on average, it's pretty good. But very few spots have the average temperature, so the air conditioner pumps too much cold air in some places and not enough in others. Brightness and contrast work on a similar principle; you increase brightness by 10 percent, say, and some areas will get too bright, others will remain too dark, and only a few will be just right.

To properly adjust the tonal values in an image, you need to modify how they're distributed, not just shift them all by a fixed amount. A histogram tells you how many items in your data fall into a user-defined set of categories. In imaging, it tells you how many pixels in your image have a lightness, red, green, or blue value of 0, 1, 2, and so on. Histogram manipulation, usually referred to as adjusting Levels in software, lets you anchor the darkest and lightest portions of the image and shift around the distribution of values in the middle.

Shifting the midpoint marker to the left, toward the darker output values, brightens every value to the right of the new midpoint. If you look at the position of the marker relative to the output levels grayscale bar, you can see that about three-quarters of the values will now lie in the lighter area, compared to half before the adjustment.

Tip: If the software allows you to see a real-time readout of the histogram's statistics, move the midtone slider until the standard deviation reaches a maximum--it will rise as you head toward the densest concentration of values, then start to decrease as you continue in that direction. That will deliver the widest possible tonal range for that image, though it won't necessarily satisfy any aesthetic whims you might have.

If you have a single midtone threshold value, adjusting the levels does the same thing as manipulating the image's tonal curve. Levels and curves are intrinsically the same adjustments, albeit with different interfaces.

The Photoshop dialog above has two sets of readouts: one for input values and one for output values. Moving the slider toward the densest part of the histogram tells the software to take all the values on each side of this threshold and output them slightly brighter than the original, but decreasingly so as it approaches white and black. If you put input values on the x axis and output values on the y axis, each old and new value becomes a coordinate pair. Voilà! The resulting set of coordinate pairs traces out the tonal curve. Photoshop's Curves setting allows you to create multiple anchor points and adjust the values in between. Many people find this tool difficult to use. I find it imprecise; since the curve is a spline rather than a Bezier, you can't directly change the slope of the curve, just the location of individual points, which for me undermines its advantage over Levels for simple corrections. It's great for producing psychedelic special effects, though.

Unfortunately, the software developers don't use standard terminology to refer to these tools, and they all tend to work dissimilarly. Consumer image-editing products may call their features Fill Flash or Exposure; sometimes they operate like the Levels tool, sometimes like Brightness. Take, for example, the Exposure And Lighting tool in the latest version of Microsoft's Digital Image Pro (scheduled to ship in August). It offers surprisingly sophisticated tonal-curve editing tools masquerading as Brightness and Contrast.

The upcoming version of Digital Image Pro offers a surprisingly sophisticated tonal-curve editing tools masquerading as Brightness and Contrast.

Microsoft's controls work in percentages (the sliders go from -100 to +100), while Photoshop operates completely differently. The midtone input value Photoshop reports is a bit misleading; since it's positioned between the 8-bit values for black (0) and white (255), you'd expect it to define a midtone gray value as an 8-bit value as well. Instead, that number is the slope of the curve.

And what do you know--that slope is directly analogous to gamma. (Geek aside: The curve's equation is the input value raised to an exponent, which is the same as the typical gamma adjustment when you assume the black-level error is negligible.) Gamma adjustment maps input values to output values for different hardware devices in order to produce the widest tonal range possible. By editing curves, you change the image; by adjusting gamma, you change the way a device renders the image.

There's a whole technical morass you could fall into for a complete discussion of gamma, but simply knowing what it is suffices for most practical applications. If you'd like to read more, check out Charles Poynton's excellent Gamma FAQ. And while you're reading, think of me--huddled in a blanket and wishing that it was as simple to fine-tune our room temperature as it is to fix an image's tonal range.