Light Meter
Your camera has a built in light meter that measures the light reflected off of objects within the frame. There are four standard types of metering found in digital cameras. You may have some or all of them in your camera.
Matrix (Evaluative) Metering – Is a complex metering system whereby a scene is split up into a series of zones. Overall exposure is based on evaluating each zone individually and taking an average of the total light readings. After the light is measured, the camera determines the best exposure. Matrix metering is effective for scenes and subjects that have even lighting.
Center-Weighted Metering – Exposure metering is averaged over the entire frame with emphasis placed on the central area. Center-weighted metering is used when it is important to have the central portion of the scene properly exposed.
Spot Metering – Spot metering covers just approximately four to 5 percent of the viewfinder area. It takes a precise exposure reading only at the very center of the frame and disregards the rest. A spot meter is used when there are extremes in the brightness of a scene (high contrast scenes), such as when a subject is back lit or has bright light on it and the background is dark.
Partial Metering – Partial metering is similar to spot metering but covers a larger area of the viewfinder, about 13.5 percent. It is useful for taking portrait photos when the subject is back lit. Partial metering is also often used when a scene has bright or dark areas around the edges of a subject. Underexposure is minimized by metering on the main subject in a scene.
Camera meters are color blind. They see everything in shades of grey – from black to white and every shade in between. Halfway between white and black is middle grey. Middle grey reflects 18% of light. It was determined some time back that reflected light from an average scene (think grass, trees, some sky and perhaps a person or two) was, when averaged, equal to the reflected light from an 18% gray card. Since scenes vary in the amount of light reflected, for example, a pile of snow reflects more light than a pile of coal, there is a need for the photographer to adjust their exposure when it is based on reflected light. All reflected light meters used in camera bodies reference 18% gray as a normal exposure.
So which mode should you use?
The answer is – it depends on the subject, the light direction, etc. Choose matrix or evaluative metering for evenly lit scenes. Use Center-weighted for scenes that have high contrast and where you want the primary subject to be properly exposed. Use Spot metering for back-lit subjects until you get very familiar with metering, use the spot meter for back lit subjects.
Auto Exposure Lock (AEL)
The Auto Exposure Lock (AEL) freezes the camera’s exposure settings so that if the subject is moved within the frame the camera’s auto exposure won’t change the aperture or shutter speed values. There are many situations where this may be of value to a photographer. For example, a photographer taking a portrait may want to place the subject off center. By locking the exposure (along with the focus) the image can be recomposed and the exposure won’t shift if the background is lighter or darker. What AE Lock does is simple: It “freezes” the camera’s exposure settings, so that if the camera is moved from one area to another, the auto exposure system won’t change aperture/shutter speed values. There are many situations where this may be useful. A photographer shooting a portrait, for example, might want to place the subject off-center. Taking a meter reading off the subject, locking it (along with focus), and then moving the camera to re-compose the subject means that exposure won’t shift if the background is lighter or darker.