The luminance histogram is your main guide to exposure. Discussing the luminance histogram can be problematic because, despite what you may have read, there's no such thing as an "ideal" histogram. The luminance histogram is merely a graphical representation of the brightness tones in your image. Let's take a closer look. Figure 4 shows a partial closeup of the histogram in Figure 3.
A luminance histogram is a bar graph. The x axis (the bottom line) shows brightness levels in a range starting at zero (pure black) on the left and finishing at 255 (pure white) on the right. The y axis shows the number of pixels of each brightness value that the photo contains.
The left side of the histogram shows the darkest tones in the photo, and the right side shows the lightest. The midtones occupy the center of the histogram.
I hope you now can understand why there's no such thing as an ideal histogram. An image with lots of dark tones will have spikes on the left side of the histogram. An image with lots of light tones will have spikes on the right. You should bear these rules in mind when evaluating your histograms. Experience plays a part, and you'll get better at evaluating histograms with practice.
The image in Figure 5 is interesting because it's composed of mainly white and black, confirmed by the spikes in the corresponding histogram in Figure 6. The narrow spike on the left represents the dark tones in the photo (the number 226) and the wide spike on the right represents the bright tones (the background).
Figure 5 This image is composed largely of just two tonesthe black figures and the white background.
Figure 6 The spikes in the luminance histogram correspond to the dark tones (the numbers) and the light tones (the background) in the image.