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Histograms for Digital Photography
There is nothing special about a histogram ~ it’s just a fancy name for a bar chart. You can show one on the back of your digital camera to let you know how the pixels in your image are distributed: each bar shows the number of pixels at that colour intensity level. A lot of pixels on the left-hand side of the graph means you have plenty of shadow detail. Same for the mid tones (shown in the middle), and highlights - (you guessed it - shown on the right-hand side of the graph). Obviously, if the histogram can show there is enough exposure on any part of the image, it can also alert you to any imbalances. That is useful! A “good” exposure generally looks like a hill rising up in the middle of the chart; but not always - various patterns give valuable information about the image. Your camera's exposure meter deals with exposure too, but with a histogram you can determine things like whether or not the image is lit flatly or is very contrasty. That’s not something even a hand-held meter can tell you. Histograms illustrate how each of the 256 possible levels of brightness are distributed across the image for each pixel.
The base of the graph (the horizontal axis) shows the range of brightness from zero on the left side of the graph (the shadows) to 255 on the right (the highlights). Since these are the only values your camera can capture, the horizontal line represents the camera's dynamic range and the distance occupied by the bars, left to right show the actual dynamic range of the image. The height of the vertical axis represents the number of pixels that have one of the 256 brightness values. The higher the line above the horizontal axis, the more pixels there are at that level of brightness. In other words, the vertical line represents an increasing amount of digital information from bottom to top. Reading the Histogram - The histogram on your camera’s LCD tells you whether you captured enough information to get a good image; in fact, it is more trustworthy than the visual image on an LCD screen, which can be easily affected by ambient light or in-camera adjustments. If the ‘hump” of the histogram is on the far right, you have probably blown out the highlights and you need to re-shoot at a higher shutter speed or a smaller aperture aperture. If the graph is shifted to the left, the opposite is true. Peaks on the left and on the right indicates that there is not much information in the mid-tones; while very low graphs indicate a flat image. Of course, sometimes you want an “unbalanced” histogram: photos of fireworks against the night sky are mostly black with very small areas of bright light; your histogram should be weighted towards the left. Conversely, a snow scene will bias the histogram to the right; if you are taking a high-key portrait, a right-biased histogram suggests that you have got it right!
Evaluating Histograms - You use histograms to determine if there is enough detail in the highlights, mid tones and shadows of each image.So long as the the dark areas aren't right up against the left hand side and the light areas aren't hard up against the right hand side of the histogram, the subject falls within the dynamic range that can be captured by your camera. In the Editor - When you view the image in your editor(Photoshop, PaintShopPro and others), you can evaluate the dynamic range of each photo on another histogram: in the Levels dialogue, it is represented by the distance between the image's "black point" (the darkest portion of your image) and its “white point” (the brightest highlight). The three triangles on the input side of the dialogue box and two more on the output side of the Levels dialogue enable you to adjust the distribution of brightness in your image. Moving the black triangle on the left hand side (to the right) darkens the image shadows. The right hand (clear) triangle lightens the image highlights as you drag it towards the left. Dragging the middle triangle (grey) to the left or right lightens or darkens the image mid tones. There are two more triangles in the output side of the dialogue box. They have nearly the opposite effect of the triangles located above. Dragging the left (all black) triangle to the right lightens the image shadows. Dragging the right (clear) triangle to the left darkens the image highlights. Conclusion - Histograms are a “new” tool for most photographers for taking pictures and photo processing. Using them can add greater control and confidence to your photography. I encourage you to use the histogram feature in your camera after every frame. It's one of the greatest innovations offered by digital photography. |
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Very informative, thank you. :)
Interesting intel. I'm also into digital photography I have been in this field for decades now. Anyways I'm glad I come across your intel. It's very informative!
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This intel was contributed by David Rich
David Rich
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May, 2012
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