Figure 3.7. Histogram of a well-exposed deep-sky image as displayed on the Canon XTi (400D).
range) and possibly the exposure time. Under a bright suburban sky, 30 seconds at f /4, ISO 400, will record star fields well.
To judge an exposure, look at the LCD display just the way you do in the daytime. Better yet, get the camera to display a histogram (Figure 3.7) and look at the hump that represents the sky background. It should be in the left half of the graph, but not all the way to the left edge. Increasing the exposure moves it to the right; decreasing the exposure moves it to the left.
When viewing the histogram, you also see a small preview image in which overexposed areas blink. It is normal for bright stars to be overexposed. For more about histograms, see p. 169.
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