Fixing Exposures with Histograms

Trick Photography And Special Effects

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While you can often recover poorly exposed photos in your image editor, your best bet is to arrive at the correct exposure in the camera, minimizing the tweaks that you have to make in post-processing. However, you can't always judge exposure just by viewing the image on your 40D's LCD after the shot is made. Nor can you get a 100 percent accurately exposed picture by using the 40D's Live View "exposure simulation" feature described in Chapter 3. Ambient light may make the LCD difficult to see, and the brightness level you've set can affect the appearance of the playback image.

Instead, you can use a histogram, which is a chart displayed on the EOS 40D's LCD that shows the number of tones being captured at each brightness level. You can use the information to provide correction for the next shot you take. The 40D offers two histogram variations: one that shows overall brightness levels for an image and an alternate version that separates the red, green, and blue channels of your image into separate histograms.


To view histograms on your screen, press the Info button while an image is shown on the LCD. Keep pressing the button until the histogram(s) are shown. The display will cycle between several levels of information, including flashing highlights and two histograms. One histogram shows overall brightness levels, while the second type of histogram screen shows both brightness levels and levels for each of the red, green, and blue channels. During histogram display, you'll also see a thumbnail at the left side of the screen with your image displayed. Overexposed areas (shown in black in Figure 4.12) will blink, which is your prompt to use negative exposure compensation. (See Making EV Changes, above.) To change your default histogram type from Brightness to RGB, use the Histogram setting in the Playback 2 menu.

Figure 4.12

A histogram shows the relationship of tones in an image.

Figure 4.12

A histogram shows the relationship of tones in an image.

Both types are charts that include a representation of up to 256 vertical lines on a horizontal axis that show the number of pixels in the image at each brightness level, from 0 (black) on the left side to 255 (white) on the right. (The 3-inch LCD doesn't have enough pixels to show each and every one of the 256 lines, but, instead provides a representation of the shape of the curve formed.) The more pixels at a given level, the taller the bar at that position. If no bar appears at a particular position on the scale from left to right, there are no pixels at that particular brightness level.

A typical histogram produces a mountain-like shape, with most of the pixels bunched in the middle tones, with fewer pixels at the dark and light ends of the scale. Ideally, though, there will be at least some pixels at either extreme, so that your image has both a true black and a true white representing some details. Learn to spot histograms that represent over- and underexposure, and add or subtract exposure using an EV modification to compensate.

For example, Figure 4.13 shows the histogram for an image that is badly underexposed. You can guess from the shape of the histogram that many of the dark tones to the left of the graph have been clipped off. There's plenty of room on the

Figure 4.13 This histogram shows an underexposed image.

right side for additional pixels to reside without having them become overexposed. Or, a histogram might look like Figure 4.14, which is overexposed. In either case, you can increase or decrease the exposure (either by changing the f/stop or shutter speed in Manual mode or by adding or subtracting an EV value in Autoexposure mode) to produce the corrected histogram shown in Figure 4.15, in which the tones "hug" the right side of the histogram to produce as many highlight details as possible. See "Making EV Changes," above for information on dialing in exposure compensation.

The histogram can also be used to aid in fixing the contrast of an image, although gauging incorrect contrast is more difficult. For example, if the histogram shows all the tones bunched up in one place in the image, the photo will be low in contrast. If the tones are spread out more or less evenly, the image is probably high in contrast. In either case, your best bet may be to switch to RAW (if you're not already using that format) so you can adjust contrast in post processing. However, you can also change to a user-defined picture style (User Def. 1, User Def. 2, or User Def. 3 in the Picture Style menu) with contrast set lower (—1 to —4) or higher (+1 to +4) as required. You'll find more about using picture styles in Chapter 5.

Figure 4.14 This histogram reveals that the image is overexposed.
Figure 4.15 A histogram for a properly exposed image should look like this.

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Reasonable care has been taken to ensure that the information presented in this book is  accurate. However, the reader should understand that the information provided does not constitute legal, medical or professional advice of any kind.

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  • jonathan
    Can you multible exsposers eos 40d?
    3 years ago

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