Fine-Tuning Exposure with a Digital Camera's Metering System

Your camera's exposure metering system is a tireless friend that keeps plugging away, calculating its reckoning of the correct exposure (based on parameters you set) regardless of whether you choose to pay attention or not. It's available whether you're setting exposure manually or using one of the programmed or priority modes. You can't really turn off the exposure meter completely when the camera is on - although it might go to sleep after a few seconds of inactivity. Even then, as soon as you tap the shutter release button, a sleeping meter wakes up, looks at the current view through the lens, and reports its findings.

How does metering work?
The reason dSLR (digital single lens reflex) exposure meters are so accurate is that they interpret the actual light passing through the lens, which is flipped upward by the mirror toward the viewfinder (or sideways in the case of cameras like those in the Olympus E series). Some of the light is used for viewing, and some is used to measure the exposure. If you zoom the lens (which often changes the amount of light passing through) or place a gadget like a filter in front of the lens to modify the light, the meter sees the results and takes them into account. The metering system is also linked to the shutter speed and aperture controls, so it understands the effects of both on the recommended exposure.

Choosing a metering scheme
Metering schemes on your dSLR enable you to configure the photosensitive elements of the exposure system (which can number from a dozen or so to thousands of individual light sensors) so that the exposure system interprets the incoming light in a specific way. Table 1 explains how photographers typically use the most popular schemes.

Table 1: Metering Schemes

Scheme	What It Does	Best Used For . . .
Center weighting	This system looks at the entire frame, but tends to emphasize the portion of the image in the center, assigning a center weighting determined by the vendor, but which usually amounts to about 80 percent for the center and 20 percent for the rest of the image.	Scenes in which the most important subjects are in the center of the frame. Perhaps you're shooting portraits or close-ups of flowers and, naturally, want to center your subject. Center weighting zeroes in on those subjects and isn't influenced by very bright or very dark areas outside the center.
Spot metering	This method makes its exposure recommendations based only on a center spot shown in the viewfinder which might measure 6mm to 12mm. Illumination outside the spot is ignored. Your dSLR might allow you to choose the size of the center spot.	Subjects that don't dominate the frame, and which are surrounded by areas of misleading brightness or darkness.
Multipoint metering	This mode is the default metering mode for most dSLRs. It collects exposure data from many points on the screen (usually not shown in the viewfinder) and uses sophisticated algorithms to decide which points to use in calculating the correct exposure.	Any scenes that don't require the special treatment provided by the other two methods. In other words, you'll use multipoint metering almost

More versatility with metering options
Your digital SLR has several options that can increase your exposure versatility:

• Lock in settings with exposure lock control. When you press the button, the current exposure (or focus, or both) is locked until you press the button again or take a photo. This lock gives you the freedom to set exposure and then reframe the photo any way you like without worrying that the preferred settings will change. This option is different from the normal system of locking exposure and/or focus when you press the shutter release, partially because you don't have to keep your finger on the release button. Exposure lock control is sometimes combined with a focus lock adjustment.
• Shoot a series of photos at different exposures by setting your dSLR's bracketing system. The camera takes the first picture at the metered exposure, and then it takes the second and third at, say, one-third stop less exposure and one-third stop more exposure. You can set the exact increment, choosing to bracket by half or full stops if you want. You can also bracket parameters other than exposure, such as white balance and flash. Your camera might allow you to bracket more than one of these, and in the order you choose.
• Adjust the shutter speed and/or aperture combination in use without changing the exposure at all. If the camera chooses 1/250 of a second at f8, spinning the command dial to the right might switch to 1/500 of a second at f5.6, or to the left to change to 1/125 of a second at f11. All these exposures are the same, but they provide different useful combinations of shutter speeds and f-stop sizes.
  

source : tech.yahoo