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A histogram can tell you a lot about your image, both before clicking the shutter button and during post-processing. Here's how to read it.
There are a lot of things your camera can do for you, but one thing it doesn’t always get right is the exposure. And while most of us rely on checking the shot visually on our camera's LCD screen, it’s easy for our eyes to be misled. That’s where the histogram comes in.
The histogram is a simple graph that shows exactly what’s going on with an image’s exposure. That makes it an almost fool-proof way of getting the best exposure for your shot in camera. Of course, if you’re shooting in Raw, you can always correct the exposure later, but it doesn’t hurt to go for the best in-camera shot you can get.
The histogram is similar to a standard bell curve, with the left side representing the blacks/shadows and the right side representing the whites/highlights. The middle section is for the mid-tones. How high a line reaches represents the number of pixels in that particular tone, with the tones ranging from 0 (black) to 255 (white).
In general, the best exposure will have a graph that reaches from edge to edge, with the tones being fairly evenly distributed over the entire spectrum. A histogram with the majority of the reading on the left shows that there’s a strong emphasis on the shadows, and unless you’re going for a low-key image, your photo is probably underexposed). If the majority of the histogram falls on the right, then the highlights are emphasized and, unless you’re going for a high-key look, your photo is likely over-exposed. If there are extreme peaks on the shadow side, it means you’ve clipped the shadows and all detail in those shadows will be lost. If it peaks on the right, that means you’ve clipped your highlights and possibly even blown them out. This will erase all texture from the brightest areas of the image and looks quite horrendous when printed out.
In theory, the ‘perfect’ photo histogram will rise gently from the far left, turn into a bit of a mountain in the middle, and then drop down gently to the other end. This indicates that your image has the full range of tones, with no loss of detail in the highlights or shadows. For example, take a look at the image below. It has a perfect histogram and indeed, the image seems quite properly exposed.
The photo below, however, has the majority of its tones on the left-hand side. This shows that it’s underexposed.
This next image, taken as the last in a 3-image series for an HDR photo, is quite over-exposed, even to the point of blowing out the highlights. You can see this from the large spike in the far right-hand side of the histogram.
Of course, there are times that you want the majority of the tones in the right or left-hand side. The photo above is one example, as it will be combined with an under-exposed and properly exposed image to form an HDR photo. Other times you might want a histogram weighted to one side or the other is when shooting high- or low-key images, or simply because your image is meant to be primarily bright or dark. Take this next image, for example. The majority of the histogram is to the right because the water is bright, yet the crocodile is perfectly exposed.
While the first use of the histogram is for properly exposing your shots in-camera, it can also help you know how to correct improperly exposed photos once you get to post-processing. This, of course, works best if you’re shooting in Raw.
As you can see, all of the tones are in the middle, with a huge gap on both sides between the tones and the edge of the graph. This shows a lack of contrast, and indeed, the image looks quite flat. To fix this, we’ll need to stretch out each end of the “mountain” towards the edges of the histogram. We do this by adjusting the whites, blacks, highlights, and shadows in the Raw Develop filter.
If you look at the histogram in the corrected image, you’ll see that it spreads closer towards the edges. You’ll probably also notice that the hazy magenta color cast is gone and that the green of the forest is more vibrant. There’s also quite a bit more contrast to the image, without ever touching the contrast slider. From here you can proceed to the fine-tuning filters. In fact, if you’re new to shooting in Raw and haven’t really known what to do with all those sliders in the Raw develop module, the histogram is an excellent guide.
There’s one more important aspect of the histogram. Looking at the spikes in your histogram isn’t the only way to see whether there are clipped shadows or highlights in your image. If you look closely at Luminar’s histogram, you’ll see little triangles in the upper corners. Clicking on either triangle will highlight in red the areas that are clipped—click on the right triangle for highlights and the left for shadows.
In the end, whether you’re a beginning photographer who relies on your camera’s auto mode, or an advanced photographer shooting exclusively in manual, the histogram can be your best friend, both in the field and during Raw post-processing. And, like with most friends, the more time you spend with it, the closer you will become.
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