Let’s start with what is histogram or histogram graph? The histogram is a graphic representation of the tonal range in a photograph, and it’s analysis of the image’s tonal range provides a precise check on exposure. Does histogram really help photographers to understand their images? Yes it helps photographers to understand the tonal color structure of the photographs. Histogram can show whether your photograph is composed of dark tones, light tones or mixed color tones. Histogram can be represented with 256 color tones in digital format. The darkest color tone is represented by number 0 on the left side of the graph and the lightest color tone is represented by number 255 on the right side of the histogram graph. On the tonal graph, the horizontal axis displays color tones from the darkest on the left to the lightest on the right and the vertical axis shows the number of pixels that have each color tone has.
When you frame and compose your photograph, your camera’s light meter reads the scene before you take the photo and the histogram analyzes the photo you’ve just taken. You can choose to have the histogram appear on the camera’s LCD along with the playback display of your photo by using Multi Selector Dial button. You may be thinking why histogram is so important in photography but yes it is very important graphic tool because it helps to understand the exposure of the photograph you have taken and tells you if it’s necessary to adjust exposure. It’s not necessary to check histogram graph for each and every photo you have taken but it’s good practice to check the histogram when a scene’s lighting condition is tricky; when there are areas of deep shadow and bright light in the same scene and when you’re going to take a series of images in the same setting and want to be sure your exposure is right on target.
Now you may be thinking what can anybody get from histogram graph? Well the answer is histogram will tell you if your photographs or parts of photographs are overexposed (lack of detail in the highlights) or underexposed (loss of details in the shadows). When you look at the histogram graph and there is heavy concentration on the left side of the graph, that means the image is underexposed and you’ve lost detail in the shadow areas. Likewise a heavy concentration on the right means the image is overexposed and your highlights may be blown out. Now when you see any of these kind of situation in histogram, you can think of solution as per your condition whether it is underexposed or overexposed. If you photograph is overexposed, you can increase your shutter speed, close down aperture or lower your ISO to correct it and if your photograph is underexposed, you may want to decrease shutter speed, wide open aperture or increase ISO level little bit considering possible noise in the photograph.
Now let’s look at the couple of photographs that I took to demonstrate histogram graphs and how to understand histogram graph (please consider the quality of photographs as they are not intended to be high quality images).
You can see fair amount of pixels are shifted on the left side because photograph is composed with dark color tones (dark night) plus it is little bit underexposed and large amount of pixels are in the middle of distribution indicating there are mid-tone colors. That’s why it is good idea to shoot in RAW+JPEG mode because sometimes it’s possible to retrieve loss tones in RAW images but in JPEG mode, it’s permanently lost.
Let’s look another example which is overexposed and see how histogram indicates that problem.
Here you can see lots of spikes of pixels on the very right side of the graph indicating this photograph is overexposed and highlights are blown out destroying details in the picture. It has pixels on the left sides also because of the dark side on the left side of the picture.
You can also set the highlight overexposure warning (default in Nikon D60 I think but has to setup in D90) in your DSLR camera settings. When you set this option, the areas of overexposure will blink in the playback image and you will know exactly which areas of the image are overexposed; pretty cool feature huh!!!. Several Nikon D-SLRs feature secondary, color histograms. You can choose to display them and you’ll see three small graphs that show the intensity of the RGB (red, green and blue) color values in the scene. If you need to adjust these values, the camera’s white balance control is the way to do it. Some Nikon D-SLRs also allow you to magnify specific areas of the photo on playback so you can check exposure and detail rendering in very specific parts of the image. In effect, you are directing the histogram’s area of analysis.
Now you may want to go out of your comfort blanket and take some pictures and analyze those pictures using histogram graph. More you use histogram graph to analyze picture, more you will be familiar with what histogram can tell you about your photograph.
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