Figure 3–3 shows a scene photographed at two different apertures (but otherwise exposed the same). I shot the image on the left with an aperture of f/2. The subject stands out because the background is blurry and the image has an overall softer feeling than the one on the right. I photographed the image on the right at f/16, giving sharp detail throughout the scene and drawing more attention to the background and away from the subject itself.
50mm, 1/8 sec, f/2, 100 50mm, 1/8 sec, VIS, 100
Figure 3–3: Two images exposed the same but taken with different apertures.
Setting your shutter speed
Your shutter is a curtain that sits in front of the digital sensor. When it's closed, no light enters your camera. When the shutter opens, the digital sensor is exposed to light. The shutter speed controls the length of an exposure by determining how long the shutter remains open: The slower the shutter speed, the longer the exposure. For normal, sunny-day shooting, shutter speeds typically are measured in fractions of a second, such as 1/125 or 1/60.
When you slow down your shutter speed by 50 percent (from 1/60 to 1/30), you increase your exposure by 1 stop. If you speed up your shutter speed by 100 percent (1/60 to 1/125), you decrease your exposure by 1 stop. In normal lighting conditions, your shutter speed typically is a fraction of a second. A shutter speed of 1/125 second is a fairly common speed because it's fast enough that you don't get motion blur from the movement of elements in a scene or the shaking of your hand. Motion blur is pleasant in photographs only when used appropriately and for a purpose. (Chapter 16 tells you more about motion blur.)
In low-light situations, you may have to slow down your shutter speed in order to expose an image. If you need to use a shutter speed slower than 1/125 second, place your camera on a tripod to eliminate any camera shake during the exposure.
Shutter speed controls movement in your scene, so setting your exposure with a priority to shutter speed (rather than aperture or ISO) is essential when you need to freeze or reveal motion in an image. Generally, you shoot as fast as possible in order to reveal your subject with as much sharp detail as possible. But you also can reveal motion by using a slow shutter speed (see Chapters 10 and 16 for more information).
Figure 3–4 shows a scene that was photographed using two different shutter speeds. The figure on the left was taken with a fast shutter speed of 1/30, and the figure on the right was taken with a slow shutter speed of 4 seconds. Notice how much movement was captured with the slower shutter speed compared with the faster one.
Adjusting your ISO
The ISO setting on your camera determines how sensitive your digital sensor is to light. The higher the rating, the less time and light you need to achieve a proper exposure. On most digital cameras, the ISO ranges from 50 to 6400. Every time an ISO rating doubles, light sensitivity increases by 1 stop. Here's a list of the ISO ratings to choose from in 1-stop increments: 50, 100, 200, 400, 800, 1600, 3200, 6400.
24mm, 1/30 sec, f/4, BOO 24mm, 4 sac, f/11, 50
Figure 3–4: Changing your shutter speed can have a drastic effect on the way your scene is depicted.
With most digital cameras, you can shoot anywhere with an ISO of 50 to 400 without getting a great deal of digital noise (random speckles of red, green, and blue in an image that appear similar to film grain). Noise usually is undesirable because it makes an image difficult for viewers to read.
ISO settings higher than 400 can cause a photograph to have a noticeable amount of digital noise, so avoid shooting at those settings if you want a smooth quality to your images. The level of noise produced at a given ISO is dependent on your camera. It's a wise choice to do some testing to find out how high you can go before you begin to lose image quality from noise.
I recommend shooting with a very high ISO only when you
|j-" Are in a low-light situation and have already determined that you can't further open up your aperture or slow down your shutter speed v• Want an artistic look in which noise would work to your advantage
Figure 3–5 shows a cropped-in detail of an image I photographed with my camera's minimum ISO and its maximum ISO. I took the image on the left with an ISO rating of 50, and it appears smooth and without flaws. I took the image on the right with an ISO of 3200, and it contains a lot of noise.
Using a histogram to check exposure
A histogram is a graph that displays the brightness distribution in an image. You can use your camera's histogram to determine how your total scene is being exposed. When photographing a scene, you use the histogram to determine whether you need to increase or decrease your exposure. (Check out your owner's manual to find out how to use your camera's histogram.) Figure 3–6 shows what a properly exposed image may look like in the histogram.
50mm, / sac, f/5.6, 50 50mm, 1/60 sac, f/5.6, 3200
Figure 3–5: The appearance (and lack) of digital noise in an image.
Shadows Midtones Highlights
Figure 3–6: A histogram for a properly exposed image.
The far left of the histogram in the figure represents black without detail, and the far right of the graph represents white without detail. A full-scale exposure (one that contains a full range of tonalities) has tonalities that range from black to white with everything in between. A full-scale exposure is the ideal in most situations. Some exceptions exist, however. For example, consider a scene that's supposed to be light, such as a snowy scene. In this case, most of the information would be skewed to the right. The opposite is true for a particularly dark scene.
If your scene's tonality is weighted heavily toward the left and is bunched up specifically at the very edge of the histogram, your image is underexposed. If the scene's tonality is bunched up on the right side of the histogram, your image is overexposed.
In digital photography, detail in your highlights is vital. If you overexpose your image, the detail in your highlights is gone for good. You can't recover highlight information in postproduction if it wasn't there to begin with. So, the best way to expose a scene is to capture detail in the highlights and to brighten the shadow areas (if necessary) in postproduction. For more on postproduction editing, see Chapter 18.
Discovering your camera's light meter
Your digital camera has a built-in light meter that determines how much light is available in a scene and helps you properly expose your image. The meter reads light that's reflecting off an element in a scene, and determines (based on your chosen ISO) how the level of light affects your digital sensor. Your camera then tells you whether you're underexposed or overexposed. You can compensate accordingly by altering your aperture, shutter speed, or ISO. (Read more about aperture, shutter speed, and ISO in the earlier section "Taking a closer look at aperture, shutter speed, and ISO.") Check out your camera's manual to discover where to see its meter readings and how to change your exposure settings.
You often can choose from four different metering modes, which you can read about in your camera's owner's manual. Each mode is useful for different subjects. Here are the modes and their uses:
0 Evaluative metering: This mode detects the subject's position based on where you place your focal point. (For more on focal point, turn to Chapters 2 and 7.) It reads the value of light all around the subject and compares its brightness to the rest of the scene to determine a proper exposure. Evaluative metering usually is the standard or default mode, because it's suited for most subjects and situations.