Your camera probably does have a histogram, at least in Review Mode. However, this histogram is only shown after an image has been taken.
The CHDK has a bit more to offer. One option is showing a Live Histogram on the display. The histogram can be shown when the shutter button is half-pressed (Shoot) or always. So, with a quick look at the histogram, you can decide whether the image is correctly exposed or whether it is necessary to adjust the exposure (see section 4.3.1 for the fast Ev buttons). In the case of too much contrast, however, it would be better to shoot an HDR bracketing series (section 4.6.2). There is a lot to configure:
- Different histogram styles can be chosen in the entry Histogram Layout (Figure 4-21):
RGB Displays the sum of the red, green, and blue pixel values. Y Displays the luminance component. RGB Y Displays RGB above luminance. R G B Three histograms, one for each color. RGB all Displays all five histogram types with RGB on top. Y all Displays all five histogram types with luminance on top. Blend Blends the three histograms of R G B into one. Blend Y Same as Blend, but with the luminance histogram added below.
- Histogram Mode allows you to choose between Linear and logarithmic (Log) scaling of the histogram Y-axis. Logarithmic scaling can be useful if the image contains a large amount of similar colors. With linear scaling, this would result in very high peaks.
- When enabled, Show Histogram Over/Under EXP displays red dots at both ends of the histogram if over- or underexposed areas are detected, as shown in some histograms in Figure 4-21.
- Ignore Boundary Peaks allows clipping a specified number of pixels (032) at the left and right side of the histogram. Typically, this situation arises when the subject matter is small and placed on a monotonous background, as in night shots. A large black area would result in a high peak at the left side of the histogram, and the remaining information would vanish because of scaling. Removing these peaks makes the histogram readable again.
- Another way to deal with peaks is provided by Auto Magnify. If enabled and the histogram is filled to less than 20 percent, the Y-axis is enlarged and peaks are clipped. The clipped peaks are marked with a red dot, and the amount of magnification is displayed above the histogram.
- Show Histo Ev Grid adds four or five vertical grid lines to the histogram that indicate Ev values. The lines have a distance of one Ev (one f-stop).