Before to discuss histograms, we need to talk a little about exposure. So what is exposure? Exposure is to how much or little light your camera’s sensor is exposed. When a photo is too light we call the photo over-exposed, when it’s too dark we say it’s under-exposed. When a photo is underexposed the dark areas could be pitch black and no detail is found in these areas of the photo. An overexposed photo shows white areas where again there’s no detail. So even in post-processing the over or under exposed part can become darker or lighter, but without any details. That is why we want to correctly expose a photo, so we can see the details and shades of colours throughout the image. This is where the histogram is handy.
What is a histogram?
A luminosity histogram is a graph that shows how darks and lights are divided in your photo. There are 256 columns from black on the left to white on the right and the grey scales in between. This results in a mountainous landscape. Let’s just have a look, it’s easier to explain with an example than in words. Below are three histograms: ‘correctly’ exposed, over-exposed, and under-exposed. The ‘correctly’ exposed image has no total black or total white pixels and the mountains or hills are now in the middle of the histogram. The over exposed photo shows is histogram with the peaks on the right. As you can see the under exposed photo has most pixels in the dark part, that is on the left of the histogram.
How to use the histogram
When you take a photo the camera already indicates if the exposure is correct. Depending on your metering mode this will be the average of the whole or a part of the sensor. Since this is an average, there can still be areas that are blown out (completely white) or completely black. The histogram however gives you a much more precise indication. You will need to switch to Live view to see this on your screen. See your camera’s manual on how to make it appear as this is not the default setting. Now you can take care to take a photo that is not over or under exposed.
You can also see the histogram after you have taken the photo. Now you can check if you’re happy with the exposure and if not, you can take another shot.
There are of course situations where over or under exposure is fine. Photography is all about creativity, so if you want to a high key portrait with the background all white, please take that photo. As long as you know what you are doing then it’s perfectly good.
A few tips
Expose to the right
In general we say that we should expose to the right (ETTR), especially in landscape photography. This is done because in post-processing more noise will become visible when brightening the shadows (the darker parts). Exposing in such a way that the histogram almost touches the right end of the histogram makes that the darker parts are as light as possible (and thus minimizing the noise). Darkening in post-processing does not create noise while brightening does. This method can only be done by checking the histogram. So as light as possible without over exposing.
Under expose in bright light
If you don’t want to look at the histogram before taking your photo, on a bright day, you could better under expose a little to avoid that parts of the photo will be over exposed. I would still advice to check the histogram at least after taking the photo, just to make sure.
Is your subject backlit?
If you are taking photos of a subject against the light, then forget about the histogram. This often happens if you take a photo of a person or when taking a photo indoors towards the window.
Choose the spot metering mode, which will measure the light only in a very small part of the photo and set the exposure for that part of the photo. Now your subject will be correctly exposed, but the background or the window will be over-exposed. If the area you want to expose for is not in the center of the photo you might have to lock the exposure with the asterisk (*) button on your camera.
Shoot in Raw
If you take photos in Raw format, there is actually a little bit more information then that what is shown in the histogram.This means that you will still be able to recover some highlights or blacks that where over or under exposed. This is another reason to shoot in Raw, it will give you the maximum possibilities to use your creativity in post processing.
I hope this was useful. Histograms can be complicated at first, but with a little practice are very useful to correctly expose your photos. Histograms are maybe not very exciting and might even slow you down (which I find is a good thing…) when taking photos, but you can be sure to come home with more photos that you will want to keep.