Have you ever seen this funny little graph that looks like a mountain range pop up on the back of your camera when you are taking a photograph? You may have seen it while editing a picture in Photoshop or other picture editing programs. You may have even heard the word whispered in hushed tones, but don’t know what or how it relates to your photograph.
There’s no need to fear it as part of some secret photographic society or conspiracy theory. Like the Da Vinci code, there is a key to understanding it, and once you know, you’ll wonder what all the fuss was about in the first place!
So What is a Histogram?
The histogram is simply a distribution graph of the pixels and tones in each image.
Running left-to-right along the bottom of the graph is every tone from pure black through gray to pure white. Running up the side is a pixel count. The image is broken down into tones, and the pixels in each tone are counted. The results, presented as a graph, are known as the histogram.
A visual representation of the image is most helpful, as you can see at a glance if you have a problem – the peaks on the graph show that there are more pixels in that tone than the flatter lower troughs. In other words, it shows how well exposed your photograph is.
It’s All About Exposure
The extreme left of the scale is the pure black point (shadows), which is given a value of 0 because there is absolutely no light or color. The extreme right shows the pure white point (highlights). It is given a value of 255 when Red, Blue and Green (RGB) light combine in equal quantities at a certain brightness level. The middle section of the histogram shows the mid-tones of your image.
Although there are no values higher than 255 or lower than 0, if the brightness level is too high or too low, then you will see a large graph spike that ends above the horizontal line at one end or the other of the histogram.
This known as clipping. Clipping happens when there is too much or too little light for the camera sensor to record detail in a part of the image. If this happens, you may need to adjust your exposure.
Why You Should Use the Histogram
So why should you use the Histogram? After all, you can check the picture on the back of your camera right?
The truth is, review screens do not give a true representation of your photograph. It can make images appear lighter or darker, add extra contrast and color, so you can’t trust fully what you see there.
By viewing the histogram, you can tell whether the picture you are taking is under, over or well exposed.
A well exposed image has a balanced spread of tones all across the histogram, including some pure black and pure white tones. If the histogram doesn’t extend to both ends, then your photograph may be missing some contrast and your picture may look ‘flat’ – that is, a bit lifeless with no punch or visual impact.
The exception is high and low key photography. High key images have very bright, white backgrounds, while low key images have areas or backgrounds of dark tones or shadows. Using the Histogram, you can be sure you are getting the correct level of high or low key elements into your images.