To fully interpret what the exposure meter on your Nikon D3100 tells you, you need to know which metering mode is active. The metering mode determines which part of the frame the camera analyzes to calculate the proper exposure. The metering mode affects the exposure-meter reading as well as the exposure settings that the camera chooses in the fully automatic shooting modes (Auto, Portrait, and so on) as well as in the semi-auto modes (P, S, and A).
Your D3100 offers three metering modes:
Matrix: The camera analyzes the entire frame and then selects an exposure that’s designed to produce a balanced exposure.
Your camera manual refers to this mode as 3D Color Matrix II, which is simply the label that Nikon created to describe the specific technology used in this mode.
Center-weighted: The camera bases exposure on the entire frame but puts extra emphasis, or weight, on the center of the frame.
Spot: In this mode, the camera bases exposure entirely on a circular area that’s about 3.5mm in diameter, or about 2.5 percent of the frame. The exact location used for this pin-point metering depends on an autofocusing option called the AF-area mode.
If you choose the Auto-area mode, in which the camera chooses the focus point for you, exposure is based on the center focus point.
If you use any of the other AF-area modes, which enable you to select a specific focus point, the camera bases exposure on that point.
Because of this autofocus/autoexposure relationship, it’s best to switch to one of the AF-area modes that allow focus-point selection when you want to use spot metering. In the Auto-area mode, exposure may be incorrect if you compose your shot so that the subject isn’t at the center of the frame.
You don’t have a choice of metering modes in any of the fully automatic exposure modes, including Guide mode; the camera automatically uses matrix mode for all shots. But in P, A, S, or M modes, you can specify which metering mode you prefer by using either of these techniques:
Quick Settings display: Bring up the Shooting Info screen and then press the Info Edit button to shift to the Quick Settings display. Highlight the Metering icon and press OK. Select the desired mode and press OK again.
Shooting menu: Select Metering to access the available settings.
The metering mode you choose stays in effect when shooting in P, S, A, and M modes, even when you turn off the camera. Remember to change the metering mode when taking a picture under different lighting conditions.
