What aperture should you use? Your f-stop will be based on the ISO you select.
You might think that because the sky is so dark you need a wide aperture. Just the opposite is true. Remember, your objective is not to record the dark sky except as background. You want to record the intensely bright streaks of color. Were you to use a wide open aperture during your time-exposure, you would probably overexpose the colors. Result: They would “burn out” and lose coloration. To intensify the color, therefore, use a smaller aperture like f/8, or f/11, or even f/16. As with your choice of shutter speed, you will have to set your aperture manually. Which you should use depends upon your digital camera’s ISO setting (or the speed of your film), and the intensity of the color bursts. We suggest you bracket your shots, using different apertures.
Using one of the suggested apertures listed below, you can use your preview to test and then compensate the aperture accordingly.
ISO SETTING APERTURE RANGE
ISO 100 ƒ/8 to 16
ISO 200 ƒ/11 to 22
ISO 400 ƒ/16 to 22
This chart will work with most digital cameras that allow you to set shutter speed and aperture. Most of the sophisticated digital point-and-shoot models permit the photographer to set these controls. If you’ve never done this before, you’ll have to figure out how to use these controls by looking at your camera’s instruction book. If you’re using a digital SLR, then try these settings too. Naturally, you’ll check your results by reviewing your initial photos on your camera’s LCD panel, until you get the exposure that looks best.
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