Digital Photography

My previous post, which introduces you to the “Big 3″ of digital camera exposure controls — aperture, shutter speed, and ISO – explains that you can achieve the same image brightness by using many different combinations of these settings. (If you haven’t read that post, scroll to check it out now, or the rest of this probably won’t make much sense.) I also alluded to the fact that each of these settings impacts your photograph in ways that go beyond mere image brightness.

Understanding these side effects, in my opinion, is one of the most important things you can do if you want to shift from mere picture-taker to photographer. Why? Because through your choice of aperture setting (f-stop), shutter speed, and ISO, you can put your creative stamp on a scene and control picture quality in the following ways:

• The aperture setting (f-stop) affects depth of field. A low f-stop number produces a shallow depth of field. That means that your subject is in sharp focus, but objects at a distance appear blurry. A high f-stop number enlarges the zone of sharp focus so both your subject and distant objects appear sharp.
• Shutter speed determines whether moving objects appear blurry. A fast shutter speed “freezes” a moving object; at a slow shutter speed, the object appears blurry. A slow shutter speed also causes all-over image blur if you move the camera during the exposure.
• ISO affects image noise. Noise is a defect that makes your image appear speckled. As you raise the ISO, you increases the chances of noise. (The defect is similar to what you see when you shoot film pictures using high ISO film.) For this reason, you should always use the lowest ISO setting possible given the current lighting conditions.

The two pictures here, taken from my book Canon EOS Digital Rebel XSi/450D For Dummies, offer one example of how you might choose different aperture and shutter speed settings depending on your creative goal. I took the left picture using an aperture of f/13, a shutter speed of 1/25 second, and ISO 200. For the right picture, I again used ISO 200 but reduced the f-stop setting to f/5.6. That in turn reduced depth of field, so the background building is less sharply focused than in the f/13 shot. Of course, lowering the f-stop opens the aperture, letting more light into the camera, so in order to maintain the same exposure, I had to either lower the ISO (making the camera less sensitive to light) or use a faster shutter speed, shortening the exposure time. For this example, I choose the latter, raising the shutter speed to 1/125 second. This choice, too, affected my picture because the scene contains motion — the spray coming from the fountain. Notice that at a shutter speed of 1/25 second, the water appears blurrier than in the 1/125 second shot. (Using a very slow shutter speed is how nature photographers produce those “misty waterfall” images.)

Now that you have an idea of the possibilities, it’s time to find out how much your camera enables you to control aperture, shutter speed, and ISO. The answer depends on how many exposure modes your camera offers. Tune in tomorrow to get a rundown of the most common exposure modes and the level of control each offers. Even if you have a pretty basic point-and-shoot compact camera, you may have more flexibility than you know.

Exposure

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