When you preview pictures in your camera, you might see some sort of graph beside each one. If you’re a beginner, you’re probably wondering what it means.
That graph is what we call a histogram.
We use the camera histogram as an indicator of how well a picture is exposed. You can view it as a graph with the x-axis representing the range of tones or pixel brightness, while the y-axis represents the number of pixels.
I wouldn’t worry about it too much in this course, at this stage. For now, what you need is to understand the basics, which I quote from the manual of my camera.
1. If the image contains objects with a wide range of brightnesses, the distribution of tones will be relatively even.
2. If the image is dark, tone distribution will be shifted to the left.
3. If the image is bright, tone distribution will be shifted to the right.
4. While the previous three are your typical exposures, take note that it is also possible to get underexposure and overexposure in the same picture.
Histograms can actually get much more complicated, especially with cameras and image editing software being capable of plotting for each color channel and all that. But you don’t need to worry about those for now. This is as far as I expect you to know in this course with regards to histograms.
Keep in mind that there are no rules as to what type of graph is best. Histograms of the same picture might not even be consistent across different cameras and software applications. All a histogram does is provide us with a rough estimation of exposure. It’s ultimately up to you to decide if your picture is too light, too dark or just right.
You can read about it in-depth in articles such as this. Ken Rockwell also wrote a pair of brief and easy to understand tutorials here and here.