Understanding exposuresetting side effects

Trick Photography And Special Effects

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As illustrated by the images in Figure 5-4, you can create the same exposure with different combinations of aperture, shutter speed, and ISO. And although the figure shows you only two variations of settings, your choices are pretty much endless — you're limited only by the aperture range allowed by the lens and the shutter speeds and ISO settings offered by the camera.

f/13, 1/25 second, ISO 200 f/5.6, 1/125 second, ISO 200

f/13, 1/25 second, ISO 200 f/5.6, 1/125 second, ISO 200

Figure 5-4: Aperture and shutter speed affect depth of field and motion blur.

But the settings you select impact your image beyond mere exposure, as follows:

^ Aperture affects depth of field. The aperture setting, or f-stop, affects depth of field, which is the range of sharp focus in your image. I introduce this concept in Chapter 2, but here's a quick recap: With a shallow depth of field, your subject appears more sharply focused than faraway objects; with a large depth of field, the sharp-focus zone spreads over a greater distance.

As you reduce the aperture size — or stop down the aperture, in photo lingo — by choosing a higher f-stop number, you increase depth of field. As an example, notice that the background in the first image in Figure 5-4, which I shot using an aperture setting of f/13, appears noticeably sharper than in the right example, which was taken at f/5.6. Aperture is just one contributor to depth of field, however; see Chapter 6 for the complete story.

^ Shutter speed affects motion blur. At a slow shutter speed, moving objects appear blurry, whereas a fast shutter speed captures motion cleanly. Compare the fountain water in the photos in Figure 5-4, for example. At a shutter speed f/29, 1/5 second, ISO 200

of 1/125 second, the water droplets appear much more sharply focused than at 1/25 second. At the slower shutter speed, the water blurs, giving it a misty look. How high a shutter speed you need to freeze action depends on the speed of your subject, of course.

If your picture suffers from overall image blur like you see in Figure 5-5, where even stationary objects appear out of focus, the camera itself moved during the exposure. As you increase the exposure time (by selecting a slower shutter speed), you increase the risk of this problem because you have to keep the camera still for a longer period of time. Most people enter the camera-shake zone at speeds slower than about 1/50 second, although some people have steadier hands than others Figure 5-5: Slow shutter speeds increase the risk of all-over blur caused by camera shake.

My abilities vary depending on the day and my caffeine intake; I was able to snap the first example in Figure 5-4 at 1/25 second, but frankly, that was a lucky accident as I usually can't handhold at speeds that slow. At the 1/5 second used in Figure 5-5, camera shake was almost inevitable.

To avoid this issue, use a tripod or otherwise steady the camera. And see Chapter 6 for tips on solving other focus problems and Chapter 7 for more help with action photography.

^ ISO affects image noise. As ISO increases, making the image sensor more reactive to light, you increase the risk of producing a defect called noise. This defect looks like sprinkles of sand and is similar in appearance to film grain, a defect that often mars pictures taken with high ISO film.

Ideally, then, you should always use the lowest ISO setting on your camera — 100 — to ensure top image quality. But sometimes, the lighting conditions simply don't permit you to do so and still use the aperture and shutter speeds you need. As an example, I shot the rose images in Figure 5-6 on a windy day. Even after I opened the aperture to f/5.6, the maximum possible for the lens I was using, I needed a shutter speed of 1/50 second to expose the image at ISO 100. Because the flower was moving quite a bit in the wind, 1/50 second was too slow to capture it without blur, as shown in the left image.

Fortunately, you usually don't encounter serious noise with the Rebel XTi/400D until you really crank up the ISO. You'd be hard pressed to find noise in the ISO 200 example in Figure 5-6, for example. But take a look at the examples in Figure 5-7, taken at ISO 800 and 1600. You may be able to get away with ISO 800 if you keep the print or display size of the picture small — as with other image defects, noise becomes more apparent as you enlarge the photo. Noise also is more problematic in areas of flat f/5.6, 1/50 second, ISO 100 f/5.6, 1/80 second, ISO 200

f/5.6, 1/50 second, ISO 100 f/5.6, 1/80 second, ISO 200

Figure 5-6: Raising the ISO enabled me to increase the shutter speed and avoid blur.

f/5.6, 1/320 second, ISO 800 f/5.6, 1/800 second, ISO 1600

f/5.6, 1/320 second, ISO 800 f/5.6, 1/800 second, ISO 1600

Figure 5-7: Very high ISO settings usually produce "noisy" images, which appear speckled.

color. When you bump ISO all the way up to 1600, however, expect to see noise throughout the image, as in the right photo in the figure.

Just to give you a better look at how ISO affects noise, Figure 5-8 offers magnified views of an area of my ISO 100, 200, 800, and 1600 images, plus an additional shot captured at ISO 400.

One more important note about noise: A long exposure time — say, 1 second or more — also can produce this defect. Your camera has a built-in noise-reduction filter that aims to compensate for long-exposure noise; see the sidebar "Dampening long-exposure noise" elsewhere in this chapter for details.

Long story short, understanding how aperture, shutter speed, and ISO affect your image enables you to have much more creative input over the look of your photographs — and, in the case of ISO, to also control the quality of your images. (Chapter 3 discusses other factors that affect image quality.)

ISO 100

ISO 200

ISO 400

ISO 100

ISO 200

ISO 400

ISO 800

ISO 1600

ISO 800

ISO 1600

Figure 5-8: Noise becomes more visible as you enlarge your images.

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