Preventing Blown-Out Highlights and Plugged-Up Shadows
Because the height of the bars in the histogram indicates how prevalent each shade is within the image, tall spikes on the ends of the histogram indicate that the image contains large quantities of white or black (see Figure 7). That's usually an indication of a lack of detail in the brightest or darkest areas of the image. If the image contains shiny areas that reflect light directly into the camera (shiny metal or glass, for instance), it's okay if those areas end up with no detail. But if that's not the case, part of the curve must have topped or bottomed out. You should think about moving that area of the curve away from the top or bottom so you can get back the detail that was originally in that part of the image (see Figure 8).
Figure 7 This histogram indicates a lot of black in the image because the slope is heavier on the far left.
Figure 8 After an adjustment, the histogram shows less black and a more even shape.