Sony DSC-W80/W Sony® Guide to Digital Photography (Spring 2007) - Page 20

Taking your best shot CAMERA CONTROL photography. In Monitor AF, the camera is always "pre-focusing" and doesn't lock until you press the shutter halfway. Thanks to prefocusing, the camera is faster to take the shot, improving your chances of getting exactly the shot you want. And Monitor AF works together with Multi-Point AF to intelligently capture your subject. • Continuous AF is ideal when the subject may be moving towards or away from you. It's especially useful for unpredictable subjects such as sports, pets and little kids at play. Not only is the camera always "pre-focusing," but the camera also continues to track your subject right up until the moment of exposure. Flexible Spot AF Enables you to move the focus spot almost anywhere in the frame, useful for tripod shooting. Manual focus Manual control when you need it. Most Sony cameras also include manual focus for five preset distances, enabling you to take control. The DSC-H7, H9, and the a100 Digital SLR go beyond this with continuously variable manual focus. NOT FOCUS power on FOCUS 1/2 press LOCK shutter released Single AF is ideal for subjects that stand still. (Sample photo for illustration purposes.) FOCUS power on LOCK 1/2 press shutter released FOCUS power on 1/2 press LOCK shutter released Monitor AF pre-focuses the shot to speed your picture-taking when you want to capture a "decisive moment." (Sample photos for illustration purposes.) Continuous AF is ideal for fast moving subjects, particularly ones moving towards you. The focus keeps tracking until the moment of exposure. (Note: Continuous AF uses center-point focusing only.) (Sample photos for illustration purposes.) 36 CAMERA CONTROL Exposure An essential part of photography, exposure determines the amount of incoming light that reaches the image sensor. Cameras control exposure by varying the lens opening (aperture) and the amount of time given to taking the picture (shutter speed). One of the creative joys of photography is that there's no single combination of aperture and shutter speed that's uniquely correct. Aperture The word "aperture" refers to both a physical part of the camera, a circular window inside the lens that opens and closes, as well as the number that describes the size of the opening. The number is also called an "f number." This represents the focal length of the lens divided by the diameter of the aperture opening. For example, in a 50mm lens, a 12.5mm opening would have a number of f4. A 25mm opening would be f2. The smaller the f number, the wider the aperture opening. Aperture openings are typically designed to increase in discrete steps, called "f stops." As you open the aperture, each additional f stop doubles the amount of incoming light. Thanks to the geometry of the lens, each f stop has an f number that's the square root of 2 times the previous f number. That's why f stops form the characteristic series of f1.4, f2.0, f2.8, f4, f5.6, f8, f11, f16 and so on. The choice of f stop affects more than simply the amount of incoming light. Wide apertures (low f numbers) result in limited depth of focus. So the background tends to blur. Narrow apertures (high f numbers) tend to keep the entire scene in focus. Maximum aperture, the smallest f stop at which the lens can be used is an important attribute of lens quality. Lenses that are capable of a very wide maximum aperture (f1.4, for example) are called "bright" because they admit plenty of light and "fast" because they enable the photographer to work at faster shutter speeds. These lenses also tend to cost more. Shutter speed Cameras use a shutter to control the duration of exposure. This duration is called the shutter speed, and is typically a fraction of a second. Shutter speed ranges are typically designed to double or halve the speed with each step. That's why shutter speeds typically form a series such as 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/120 and so on. As with aperture, the choice of shutter speed affects more than the simple amount of incoming light. Fast shutter speeds tend to freeze motion. Slow shutter speeds tend to exaggerate motion, in the form of blur. Exposure Value The basic unit of exposure is the Exposure Value (EV). An increase of +1 EV represents a doubling of the incoming light, either by doubling the time the shutter stays open or opening up the aperture by one f stop. Aperture + shutter = creativity Each f stop doubles or halves the amount of light. Each step of shutter speed adjustment does the same. In this way, you can choose from several aperture/shutter combinations that result in the same total value of incoming light. The result is more than just proper technical exposure. It's creative freedom. For example, in portraits, a combination of open aperture and fast shutter speed will tend to blur the background, throwing more attention on your subject. In sports photography, the same exposure value might combine a stopped-down aperture and a slow shutter speed to create a beautiful motion blur. ISO sensitivity In film, a third method of managing exposure is the choice of film "speed" or ISO sensitivity. "Fast" film with a high ISO number is better suited for shooting in low light, at some cost in picture grain. In a similar way, digital cameras offer ISO sensitivity settings. These use electronic gain to increase the image brightness at some cost in picture "noise." Typically each doubling of ISO sensitivity rating equals +1 EV. This enables advanced photographers to optimize the image by adjusting shutter, aperture and ISO all at once! Auto Exposure (AE) Of course, Auto Exposure (AE) systems can make all the exposure decisions for you. Sophisticated light metering and processing can measure the light levels from your subject and choose appropriate exposure settings. • Backlight and highlight. One obvious difficulty in auto exposure is the case where your subject is significantly Continued on page 38. Smaller Iris Larger Iris Same Exposure Value Longer Shutter Shorter Shutter The camera's iris controls the size of the lens opening or "aperture." A larger aperture (lower "f number") means more light can reach the CCD. For a given Exposure Value, a wider aperture means a faster shutter speed (shorter exposure time). 37