No matter what mood you’re trying to create in a photograph, one thing about exposure values generally remains true: there should be pure black values and pure white values if you want your images to have pleasing contrast and reproduce well. If you can set a solid baseline of black tones and white tones—which varies with the image in question and the method of reproduction—you’ll find your images have better overall contrast and even better color rendition. Reading the numerical values of pixels in highlights and shadows is a great way to ensure blacks are black enough and whites are white enough. If nothing else, setting only these values at each end of the spectrum will help prevent images from looking blah, muddy and bland.
To test your black point and white tones, open an image in Photoshop, Lightroom or any image editor that provides information on the RGB values corresponding to the pixels in a specific part of image. In Photoshop, this is done by selecting the eyedropper tool and positioning it over a particular value in a scene, then reading the RGB numbers as they appear in the Info palette.
For instance, drag your eyedropper over the deepest, darkest shadow in a scene. (It should be a fairly small area, as it’s representing the total loss of detail, the very blackest of black.) Checking the values in the Info Palette you should see that the RGB numbers are very low, less than 15 for sure. If the numbers are much higher, your black values aren’t especially black. Ideally, the darkest tones in an image should read close to pure black, which would be a reading of 0. But since 0 represents the total and complete loss of information, a better general purpose target for a good solid black is 10. At the other end of the spectrum, the lightest tones in the image should read close to pure white, which would be 255. A better target for a variety of print outputs is 245. (In each case, if an image is going to be reproduced on a glowing screen, a value closer to the extremes of 0 and 255 may be preferable.) Drag your eyedropper tool to the brightest area of the image and keep an eye on the RGB values in the Info palette.
Let’s say you discover your white values measure only in the 200 range. That tells you that the area you want to read pure white (or close to it) is actually reading light gray. This could be a function of overexposure, or not enough contrast, or both. If your black values are too high—say 50 or so—then you know your blacks don’t look black, they look dark gray.
Bear in mind that the software you use may provide a different readout than the 0 to 255 scale. Lightroom, for instance, displays RGB values in percentages, from 0 (black) to 100 (white). The principle, of course, remains exactly the same.
How you remedy any deviations in your readings is fodder for another tutorial, as there are a number of approaches. My preference is typically to use a Levels adjustment layer in Photoshop to easily slide white and black values into place. There are lots of ways to set appropriate black and white points, but the important thing is that you employ these tools to rely on a bit of science rather than your own biased eyes.
While you’re in the Info Palette, you can check something else equally important without moving the eyedropper at all. Remember how we examined Red, Green and Blue values as a method for determining brightness? The difference in those values is a great way to measure neutrality as well. If a tone is truly neutral, each of the values will be about the same. A reading of 245/245/245 indicates a bright white without any color shift—i.e. correct white balance and neutral whites in the highlight areas. A reading of 10/10/10 represents a dark black pixel without any color shift in the shadows, and a reading of 125/125/125 represents a nicely neutral middle gray.