The very word is a mystery to new photographers, yet histograms are some of the most important tools that your camera has, and most experienced photographers use them frequently. So, what are these strange graphs, and how do we read them? How can they improve our photography?
When you take a look at a histogram, what do you see? The best way to describe it is as a mountain range within the confines of a graph. The graph shows the sensitivity of our camera’s sensor to light, the outer edges of the graphs representing the limits of sensitivity, also known as dynamic range. The mountain in the middle of the graph is a representation of the combined red, green, and blue light that was captured by the sensor when you took that particular image.
Some cameras will go further and allow you to look at histograms for red, green, and blue individually. We will explain why this might be useful later on. Going back to the graph, the left-hand edge of the graph represents the darkest possible shadows that the sensor can capture. The right-hand edge shows the highlights, the brightest areas that the sensor can capture without the pixels becoming pure white, also known as “blown out” or “clipped”.
To get the correct exposure, let’s imagine that our graph is a bucket and the light is water. If the bucket is tipped to the left, the water will pour over the left edge. This means our image is underexposed. On our histogram, this will show the mountain range falling outside of the left side of the graph.
If the bucket is tipped to the right, the water pours out the right side. This means that we have overexposed the image, seen on the histogram as the mountain range falling outside the right end of the graph. The balancing trick we need to perform is to keep all the water inside the bucket. This will give us the optimum exposure.
An Overexposed Image
If we look at our histogram and see we are underexposing, we need to increase our exposure, either by opening the aperture, reducing the shutter speed, or as a last resort, increasing the ISO. Conversely, if our histogram is leaking off to the right, we are over-exposing and need to reduce exposure. To reduce exposure, increase shutter speed, reduce aperture or decrease ISO if it is higher than the base setting.
An Underexposed Image
What we are trying to do is to keep our “mountain range” of light within the confines of the graph.
In addition to indicating our exposure, this mountain range can also tell us about the contrast of the image and the distribution of light in the image. The peaks of the mountains represent areas of the exposure that have a large number of pixels with the same tone.
The more pixels with that tone, the higher the mountain peak. If we have a single large peak in our histogram, we are likely looking at a high contrast image, perhaps an image shot towards the sun. If however our mountain range is somewhat flatter and more evenly distributed, then image is of a more even or lower contrast.
Correct Exposure, Low Contrast
The Actual Image
As we said earlier, some cameras allow you to show the RGB (red, green, and blue) channels as separate histograms. This can be useful when we have a scene that is predominately one color. This can often fool the camera’s meter, so by looking at the histogram that is most relevant to that color, we can fine-tune our exposure to compensate for any errors.
This can be particularly useful when photographing reds, which are prone to blow out even on the most recent digital cameras.
A Predominance of One Color
The original image
There is one small caveat when looking at histograms. The graph itself is, on most cameras, a representation of the jpg version of the image. Even if you are shooting RAW, the histogram is only showing the jpeg light distribution.
Knowing this means that when you are shooting RAW, you will have a little bit of extra space beyond either end of the histogram with which you can still get an optimum exposure. In other words, if the mountain range is slightly off to the right of the graph when shooting RAW, you will find that you will still be able to maintain the highlights in the image.
Understanding and learning how to use histograms can be a powerful way to improve the exposure of our images, and should be part of the learning curve of any photographer wanting to take their technique to the next level.