Instruction manual

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locked, even if you move the camera. Pressing the shutter release the rest of the
way causes the camera to take the picture.
On most cameras, when you hold the shutter button halfway a steady ‘Ready’
or ‘In-Focus’ light or symbol next to the viewfinder illuminates. If you get a
blinking (or different colored) ‘Warning’ light rather than a steady ‘Ready’ light it
can indicate one of two things: 1) there is insufficient light (e.g., from cloud cover)
for the picture, or 2) the camera thinks it’s too close to the target because of a
reflection from the window (this is why we shoot photos through an open window).
This feature is handy when shooting from an airplane, as it allows you to
focus and lock onto your target at the earliest possible moment; you can then
concentrate on framing the target to best advantage before your press the shutter
release the rest of the way to capture the photo. Another advantage to using
exposure lock is that it reduces the time between pushing the shutter release and
the camera actually capturing the photo, which is a nice feature when you are
taking a picture from a moving airplane. This time lag (called shutter lag) can be
significant with low- to medium-priced digital cameras; during this wait, the
camera is subject to turbulence and the chance of a blurred shot increases.
Some cameras have separate focus lock and exposure lock controls; the
separate exposure lock is often labeled as the auto-exposure lock (AEL). Check
your camera’s manual on how to use this feature.
2.4 Exposure Modes
2.4.1 Auto
The Automatic mode of the camera determines the optimal combination of
aperture, shutter speed, and sensitivity based on the exposure value determined
by the camera’s light metering system (discussed below). The camera’s
microcomputer usually assumes that you want to take a picture using the fastest
available shutter speed because this minimizes camera shake and the blur that
can result from a moving object. As it chooses the shortest possible shutter
speed, it is only limited by how small the aperture can go given the current lighting
conditions and ISO setting. The only adjustment you can make is the exposure
compensation (see below) to over- or under-expose the image.
Weaknesses
Most camera autofocus systems rely on contrast detection: it will focus the
lens until the image has as much contrast as possible. In other words, contrast
detection is largely a function of the available light in your scene. If the area
you’re trying to focus on is too dark, or uniformly colored, the camera won’t be
able to detect any contrast and may be unable to lock on.
For example, the Nikon D200 user manual lists several situations where its
autofocus may not work well:
• There is little or no contrast between the subject and the background (i.e., the
subject is the same color as the background)
• The subject is dominated by regular geometric patterns, such as a row of
windows in a building
• The focus area contains areas of sharply contrasting brightness, such as
when the target is halfway in the shade