What you see on your monitor, even if the monitor is perfectly calibrated, can be deceptive. What may look good on screen may have serious technical shortfalls that become evident later, especially when you go to print. Let's look at the various tools Lightroom offers to evaluate your images.

Lightroom's histogram graphically displays the 8-bit Red, Green, and Blue (RGB) values of your selected image. In the case of a RAW file, the histogram is not a reflection of the actual RAW data (which is grayscale and linear), but a reflection of the processed RGB data with nonlinear tone-mapping applied.

The histogram updates in real time to reflect changes you make to the white balance or tonal distribution. As you'll see later in the chapter, distribution of the tonal values can be made directly from the histogram or from other panes in the right panel. The Lightroom histogram is much more accurate than the one associated with your digital camera, so don't be surprised to see a difference.

Note the image in. It's fairly intuitive to figure out what the colors in the histogram represent. Gray represents pixels in all three channels: red, green, and blue. Red represents red pixels, green represents green pixels, and blue represents blue pixels. Cyan represents pixels in both the green and blue channels, magenta the red and blue channels, yellow the red and green channels. A sharp line on either end of the histogram indicate clipping, meaning there is no detail in either a shadow or highlight area with the current adjustment settings. The height of the line is indicative of the degree of clipping; higher lines represent more clipping, lower lines indicate less clipping. Gray lines represent clipping in all three color channels. The color of the line tells you which color is actually clipped, so in this example, you can see no highlight clipping, and shadow clipping in all three channels.

Figure 5-1

Every image—and every tonal or color change to that image—will produce a different histogram. Here, I increased the exposure, lightening the image and thus automatically changing the corresponding histogram. Figure 5-2 The goal is to produce a distribution of tonal values based on both subjective response and quantifiable criteria (such as highlight or shadow clipping). The histogram is not the final indicator, but it's an indispensable tool for getting you the image you want or need.

Figure 5-2

Clipping Warning Tools

If you pass your cursor over the triangle on the left at the top of the histogram (circled), and if your image contains shadow areas with no detail, your preview will light up blue in those areas. If you pass your cursor over the triangle on the right (circled), and if your image contains highlight areas with no detail, your preview will light up red in those areas. These warnings take any tonal or color adjustments you make into consideration, so your original file itself might be fine, but show clipping warnings after an adjustment.

Figure 5-3

Histogram Options

Right-click anywhere on the histogram and you'll get a contextual menu. Here you can control the visibility of tools and information that appears along with the histogram.

Figure 5-4

Hold Option (Alt) Key and Move Exposure Slider

You can get even more information about exactly which colors are clipped by holding the Option (Alt) key while moving the Exposure (circled), Recovery, or Black sliders in the Basic panel.

If you hold the Option (Alt) key while moving the Exposure and Recovery sliders, the image turns black and clipped areas appear white.

Figure 5-5

Red, green, or blue colored areas indicate clipping in one of those color channels. If more than one color channel is clipped your preview will be indicated with cyan, magenta, or yellow.

Do this with the Black slider (circled), and the image turns white and clipped areas appear black. Figure 5-6

Figure 5-6

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