Histograms are quite possibly the most under-used tool in the photographers tool set, simply because they are misunderstood. At first glance they look like a deranged pointless graph, but the truth is– they can tell you almost everything you need to know about your exposure, in a simple and easy-to-read way.
How To Read a Histogram
Histograms create graphical representations of the tones and exposure in your images. They graph the tones in black to white (or color tones in color histograms), from left to right. The vertical spikes represent a single tone; the higher the spike the more pixels exist of that tone in your photo.
Simply put, high peaks on the left side means a large quantity of darker pixels and high peaks on the right means you have more brightly colored pixels. If you have peaks in the middle it means your photo has high quantities of mid-tone colored pixels.
Why they are Important
The LCD screen on most cameras is quite small and cannot display colors and shadows in great definition—making it quite difficult to read if you have over- or under-exposed your photo. A histogram can tell you what exposure level your photo is, in an extremely detailed way. Knowing your exposure levels can help to ensure that you have a properly exposed photo, and most importantly it can do this while you are still there and the exposure can be corrected—once you get home it is too late.
A histogram can show you how to understand, observe, differentiate and tune your exposure levels while you are still shooting.
Putting a Histogram to Use
Once you understand how to read levels based on the histogram you can act on it. Whilst still on scene you can note if there is too much shadow not enough shadow, too much of one color in your photo, or if the photo is overexposed. Now that you understand what it is that needs changing you can adjust your photo using the White Balance tool, and retake it. For example if your red level has spiked disproportionately to blue and green then you need to adjust your red levels with your White Balance tool.