IMPROVING EXPOSURE:
HISTOGRAMS


WHAT THEY ARE &
HOW TO USE THEM


by Florence W Deems

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Most digital cameras have a display addition to the view screen on the camera's back that is called a histogram. Sometimes this will be displayed along with the other data, such as ISO, f/stop, shutter speed, etc, before you actually make the image; other times it will show up in the review feature. Sometimes you will have to push a button or twirl a knob to see the histogram. If you don't see one, then consult your camera's manual to see if it will display histograms and if so, how to access them.

But why do we want to see this histogram? Because it's a chart of the pixel arrangement and amounts for each image. Each image is different and therefore, its histogram will be unique to it, also. Why do we need this info? We really don't, unless we want to learn how to improve the EXPOSURE ACCURACY of our images.

Basically, the histogram is a chart. The left vertical scale shows how many pixels are showing a particular shade or tone in the image, compared to all the rest of the pixels. The horizontal scale along the bottom shows the possible tonal range, from pure black at the left to pure white at the right, with all the tones and shades of gray in between. Remember that colors also can appear very dark to very light in an image. The histogram will show you whether your image is properly exposed.

The image below is of a typical histogram.

typical histogram

You're probably asking now, why would we want to look at some dumb chart, when we have the view screen on the camera's back that shows us the image we just made? Well, sometimes it's so bright out that it's very difficult to see the image on that view screen. But you ought to be able to make out the histogram, since it has high contrast. Even if viewing conditions are just right, your camera's view finder could be displaying images consistently too bright or too dark. But viewing the histogram will give you fool proof EXPOSURE information.

You can see that practically all of the pixels are bunched up in the center, with none reaching either to the left, pure black, or to the right, pure white. In fact, there's a fair amount of space on either side. So this means that the image that this histogram charts out for us has mostly middle tones of grays/colors. An image like this will look "flat," meaning that it doesn't have much contrast between blacks and whites. We can improve contrast in the camera (look in the menu) or by changing our position in relation to the direction of the light source. Or we can do it with a post processing software, such as Photoshop or Paint Shop Pro. See the image below.

histogram two squirrels

image of 2 squirrels on post

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Next is an image that has very good contrast. It doesn't look "flat" like the squirrel image does. Below it is its histogram. You can see that the pixels are spread out over the whole range from pure black to pure white.

histogram Longwood Gardens scene

Conservancy main hall


Below is another image whose histogram shows a nice distribution between very dark (but not pure black) and pure white.

histogram white squirrel

white squirrel

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If you are shooting a scene or object, and you see its histogram look like the one below, with most of the pixels jammed up over at the left side, you will know that when you see the image, it will look very dark. If this is what you want, then your exposure is correct. Below this histogram is the image. As you can see, I was going for a black object on a black background, so this exposure is just right for that purpose.

histogram black teapot

image black teapot

But if it is NOT what you want, then you will have to INCREASE the exposure until the histogram shows that many of the pixels have moved towards the right or lighter side.

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Now we'll go to the opposite extreme - shooting a clear or white object against a white background. Think for a moment: what do you think the histogram should look like? If you thought that in this case, most of the pixels should appear near the right/light side, then you are correct. See this histogram below and its image.

histogram crystal penguin

image of crystal penquin

But if you had just shot a normal scene with normal contrast, and its histogram looked like the one above, with most of the pixels over near the right side, then you'd know that you had OVEREXPOSED the scene. So you would have to REDUCE the exposure so many more pixels spread out towards the left side. And then your scene would look right and have normal contrast.

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Suppose you see a histogram that looks like this one below:

histogram of an image

A group of pixels is bunched near the left side, meaning they're showing the darkest areas, while another group is bunched up at the right/bright side, showing the lightest areas of the image. This means that this image has VERY HIGH CONTRAST, with almost no areas of the medium gray tones. See the image below.

image

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Another point to make is that for most scenes and images, you do NOT want to see a bunch of pixels piled up against either the left or the right side. For most scenes, you do not want the darkest areas to be blocked up as totally pure black. Conversely, you don't want to see larger areas of blown out white. Because in cases like these, there will be no pixel information in either the black or the white areas. Many images will look better if we can see details in both the dark and the light areas. In post processing, you won't be able to "rescue" any details when you see the histogram showing piles of pixels at either end that ride up toward the top of the histogram. Even if you can coax out a few details in the darkest areas, chances are that you'll encounter "noise" which can look ugly.

One more point: There is no such thing as a "perfect" histogram! Some people think that if the histogram shows a "bell shaped curve," then that's the perfect exposure. Nope. Remember that scenes vary in the amounts of darks and lights and medium tonalities. Therefore, each histogram will reflect this uniqueness.

Now that you've read this article, get out your camera and practice using the histogram tool to get the right exposures.

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