Wednesday, December 3, 2008

Metering: What is it? (part 1)

This is the first in a series on using the built-in meter in your camera.

What is metering?

Metering is simply the process of using a light meter to measure the brightness of a scene to determine the proper exposure. Historically it was done with separate hand-held meters that gave you a reading of the light level; the photographer would then look this up on a chart to determine the exposure settings to use and finally dial them into the camera. In time, camera manufacturers were able to shrink them down and build them into the camera's viewfinder so the photographer could more easily set the exposure while composing the image. Now, the computers in the cameras use them to automatically set the exposure for you when you're in any mode except manual.

My old Minolta had two needles on the right of the viewfinder. A thin needle moves up and down indicating the amount of light. A second, thicker needle with a circle indicates the current exposure settings. As the f/stop and shutter speed are adjusted, the second needle moves up and down. When the thin needle is centered in the larger needle's circle, you have the exposure the camera thinks is correct.

Here's the meter on the back of my digital SLR.
Metering 1

When active, there is a little pointer that runs along the bottom of the circled ruler. The center position is proper exposure and the two sides indicate under and over exposure. Adjusting the f/stop, shutter speed and ISO causes the pointer to move from one side to the other, indicating the current exposure relative to what the camera calculates as correct. There is a similar meter on the bottom of the viewfinder.

What are the various automatic modes?

Modern cameras have an array of light sensors built into them that they use to determine the light level in various parts of the image. In all the "creative" modes, the camera uses this information to get what it calculates is the best image for that mode. In these modes, the photographer has no control over how the computer interprets the information coming from the light meters.

In the semi-automatic modes of aperture and shutter priority, the meters are used by the computer to determine the part of the exposure equation that the photographer has left under its control. But, there are three options the photographer can set to give them finer control over how the computer uses the meters in these two modes. They mostly work the same way across all vendors, but they go by different names.

Manufacturer Whole frame Point mode Center-weighted
Canon Evaluative Partial Center-weighted
Nikon Matrix Spot Center-weighted
Olympus ESP Spot Center-weighted
Pentax Multi-segment Spot Center-weighted

The first mode, Whole frame in the chart above, is called something different by each manufacturer but basically works the same in all cases. It takes multiple readings across the entire image and balances the various values to get the best exposure setting. A simplistic explanation is it sets the exposure so the brightest area is not blown out while at the same time it tries to keep darker areas from being too dark. It also tries to figure out what is a neutral grey and put it in the middle between the lightest light and darkest dark.

Point mode uses just one well defined area of the sensor to take the light reading from and expose for it. This gives the photographer control over the exposure by pointing that one area at the part of the scene they want to have properly exposed and locking in the exposure. Then they can recompose the image and click the shutter. For Canon in Partial metering mode, this well defined area of the screen is always the center of the viewfinder. For the other manufacturers with their Spot metering modes, this well defined area is the focus point that is used. This keeps you from having to use exposure lock if the focus point is correct.

In Center-weighted mode, the center of the screen is primarily used to compute the exposure with areas surrounding the center given secondary importance. The outside edges are ignored. This is frequently used in portrait photography where the person in the center is most important, but the area immediately around them also needs to be taken into consideration and the area on the outside is unimportant.

The next article will show when and how you can use these different modes. To be notified when it's published, be sure to subscribe to either the RSS feed, the e-mail list or follow the Twitter feed.
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