What Is a Histogram 
Figure 1 - Anatomy of a Photographic Histogram
*Frequency indicates the number of each tonality occurrence
A histogram is a bar graph that was originally created for statistical analysis.
As digital photography was developed, designers incorporated them into cameras
to provide a graphic representation of the digitally recorded image. However,
even today their function is not fully understood and the information they provide
is often incorrectly used.
As shown in Figure 1 above, your digital camera creates a histogram that describes the tonal range of each scene you capture. The camera's processor locates each picture element (pixel) on this horizontal scale according to its relative brightness from 0 (Black) to 255 (White). Pixels of equal brightness are stacked vertically one upon another to create lines of varying heights. The result is a graph of very fine vertical lines which can appear as a smooth curve, a series of jagged lines or a combination of both.
A histogram cannot tell you about lighting ratios, the best flash-ambient mix, or if the subject is properly exposed. That is why using a light meter, along with your camera's histogram is the best way to assure optimum results that can be easily reproduced.
Photography would be much easier if histograms had instantly recognizable good shapes and bad shapes and if there were "ideal shapes" that guaranteed perfect results. The truth is, they are just graphs. All of the histograms in this brochure accurately describe the pictures located near them. Their shapes are all very different because the tonal distribution of the image they represent is also very different. At best, a histogram is a guide, it is up to you to determine how to use it.
A histogram is a great way to judge if the brightness range of a scene will fit within the dynamic range of your camera. That is, if it fits within the confines of the left and right sides, you will most likely have an easily printable image. However, if the shape of the histogram is pushed up against either side, you have to understand the difference between a normally exposed, high-key or low-key image, and the possibility of losing a highlight or shadow detail in the scene you are trying to record.
Histograms are not typically helpful in determining the exact exposure of the subject, the effect of lighting on the subject, or variations in light mixtures. And as histograms appear in a small window on the back of your digital camera, a quick glance is often not enough to make a quality determination.
Digging deeper into histograms...
It is not always easy to make sense of a histogram showing the full tonality a of scene, so we have created the graphic examples below to help simplify the process. In Figures 2-6, the tonal steps are arranged starting with no midtones, simply black and white. Each successive figure adds midtone steps. The steps in each Figure are of equal size and shape. The resulting histograms clearly show each tone as an equal height line that is uniformly distributed across the horizontal scale.
As figure 6 illustrates, even when the number of steps is increased to a degree where the impression is a continuous graduation, the resulting histogram appears smoother but can still show all the discreet tones.
