Understanding Histograms

The histogram can be confusing to a lot of new digital photographers, but it doesn’t have to be! In this tutorial you’ll discover what the histogram is, how to read it, and why it’s important to you when shooting digital photographs.

What is the histogram?
The histogram is essentially a line graph that maps the disbursement of light and dark pixels across a digital image. It’s a representation of the dynamic range of a photo, with the black point all the way on the left hand side of the graph and the white point all the way to the right hand side.

By learning to read the histogram, you can quickly see if an image is under exposed, over exposed or has too much or too little contrast.

But why would I want to do that when I can just look at the photo?
LCD displays on even the most expensive digital cameras often display brightness, contrast and exposure of images inaccurately. This is simply due to the nature of small, back-lit LCD screens. When you’re out taking photos, the histogram is the most accurate way to check and see if your shots are exposed properly.

Ok. Fair enough…. So how do I read the histogram?
Before we get started reading the histograms themselves, let me throw in a useful tip here. Typical average quality and above digital cameras have a dynamic range of about 5-6 f/stops, so it it might be helpful to you to mentally divide the histogram into 5-6 sections. This will help you determine by how many stops you should compensate when you’re re-shooting a photo with a bad histogram.

As for reading the histogram itself, to better understand that, let’s take a look at a few examples.

Have a look at the image below. You’ll see that it’s properly exposed, with a nice degree of contrast. The whites are white. The blacks are black, and there’s a good range of mid-tones in between.

Now take a look at the histogram in the bottom right corner of the above photo. It looks balanced in regards to what we should see in the photo-a few very dark areas, a few very white, and many middle values. Now, obviously, if you’re taking a photo of something that is very bright or very dark, your histogram will be skewed toward one direction or the other, but an average, well-shot photo is a good starting point to learn from.

Now let’s take a look at the histogram for the same scene when the image has been overexposed.

As you can see, there’s little to nothing on the left side of the histogram. The image is very biased toward the right hand side of the graph and judging from the high spike at the white point, we’ve really blown the highlight and lost some image data.

As you might expect, the histogram on an underexposed image shows precisely the opposite.

Our whites and lighter mid-tones are non-existent and there’s a heavy bias toward the left hand side of the histogram. This is right in line with what we see when we look at the image-a lot of dark tones and blacks, but no bright, vibrant colors or whites.

Image histograms can tell us a lot about image contrast as well. For instance, take a look a the following photograph and its histogram.

This image has very little contrast. This is evidenced by the lack of white and dark values depicted on the histogram. Our graph starts about one stop beyond the black point and ends about one stop below the white point.

What if the image has too much contrast? If you said “then most of your image data will be at the far ends of the graph” then you’re absolutely correct.

Our white and black points on this image are very high, and our mid-tones are rather flat, indicating that the images has a lot of dark pixels, a lot of bright ones, and not much else. Unless you’re photographing a white and black checkerboard instead of a beach scene, this is a bad thing. You’d want to adjust your camera settings and shoot again.

Now that you know how to read the histogram, start putting these skills to work! It will take some practice, because not every shot should have a perfectly balanced graph. Some images should be skewed to one side of the chart or the other, but with a little practice and attention to details, you’ll quickly learn what type of histogram you should expect in given situations and then you can accurately check your images for proper exposure and contrast without having to rely on a less-than-accurate, and small, LCD display.

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