Our Tips & Tricks series aims to give beginners, amateurs and professionals alike more ways to approach different aspects of photography and videography. For each installment, we’ve enlisted the expertise of our staff and have received their feedback on method and equipment.
The histogram seems to be one of the aspects of digital photography that many beginning photographers have trouble understanding. This will be a quick reminder about what the histogram represents.
A histogram charts your exposure. The graphs represent the amount of pixels that are in shadow, midtones, and highlights. Certain histograms graph the amount of pixels that contain certain color information, but for our purposes those histograms are pretty pointless.
To illustrate, let’s take a look at a few purposefully over- and underexposed images.
Clearly this image is overexposed. You can see how the histogram in the corner is very much favoring the right side of the graph. This is because the right side represents the amount of pixels that are highlights. When a histogram graph hugs the right side of the chart, that means that pixel information has been lost in the highlights. Conversely, the left side of the histogram graphs the amount of pixels that are in shadow (or underexposed), as you can see here:
Notice how the graph heavily favors the left side. However, we can see that only a little pixel information is completely lost in the shadows, as the graph is only barely hugging the left side of the graph. This means that it would be possible to retrieve a lot of the underexposed pixel information, with varying degrees of success, if I were to have shot this in RAW. The arrow points to the area where I’m guessing that most of that pixel information is lost forever.
Another interesting thing to note about the above picture is the spike in the middle of the graph. This probably represents the midtones that can be found in what should be “white” areas, such as the sign and the window frames.
A good way to think of the histogram is that it is charting the amount of pixels by exposure based on a gray scale like this:
The X-axis of a histogram charts where on the gray scale the pixels are exposed, whereas the Y-axis charts the amount of pixels per exposure level, or luminosity. Take a look at this:
The histogram here is charting the pixels of the gray scale. Because this image is consistent in its luminosity per section, the histogram chart spikes with each brightening value of luminosity.
Let’s take a look at a “properly” exposed image’s histogram:
The majority of the pixels seem to reside in the middle, midtone section of the histogram. This is generally what you want your histogram to look like. Most photography instructors will tell you that an ideal shape of a histogram graph is the bell curve shape, but that doesn’t really matter so much, as long as the pixels aren’t hugging the walls of the histogram on either side.
Here is what the histogram alone represents in this image:
I hope this quick lesson demystifies the histogram for you beginners out there! If you have questions, please leave them in the comment section below!