First, a little history on the basics and development of your camera's built-in light meter.
Weston Master II Light Meter
Measuring the amount of available light has always been a photographic requirement. The first readily available commercial devices were not incorporated into cameras, but were hand held light meters. Meters, like the Weston meter shown above, were a mainstay for photographers for decades. In fact, many professional photographers today still rely on the precision and flexibility offered by these hand held units.
As 35mm cameras became more prolific and affordable, manufacturers enhanced many of their cameras by incorporating internal metering systems. This advancement eliminated the need to carry separate light meters and increased the speed of the picture taking process. It's debatable whether this technological innovation actually improved the process of determining correct exposures.
The new problem was, "What exactly is the meter measuring?" Using the old hand held meter, the photographer would normally "take his reading" from the light being reflected by a specific element of the image he/she was shooting. In other words, if the picture was of a landscape, the photographer would physically move to an object in the scene that was close to 18% gray and read the light being reflected from that object, or the photographer would place an 18% gray card in the scene and measure the light the card reflected towards the camera. (If I've lost you with all this 18% gray stuff, take some time to read Parts 1a and 1b in this blog titled "Exposure is Everything".)
With the new built-in light meters, photographers were more likely to compose their pictures and then take a light meter reading through the lens. So in this situation, what is the meter actually reading?
The area being measured by an Averaging Meter
The answer initially was EVERYTHING. The meter evaluated the entire image area, like the one above, and produced an exposure setting that AVERAGED all the light in a scene to provide an overall 18% gray exposure. For photographers who understood what the meter was using as its basis for determining exposure, this was not a problem. If he/she focused and metered first off an object that was approximately 18% gray (or 18% color density) to set the exposure and THEN composed the entire scene through the viewfinder, everything worked fine. Good exposures.
Unfortunately, for the majority of the public with less exposure experience, the functioning of the internal meter was misunderstood. Somewhere along the line, the manufacturers' intent and message was misconstrued to mean, "With a built-in light meter, whatever I point my camera at will be exposed correctly." That's just not true. Think about it. How could a simple 1960's light meter inside a camera know the difference between a snow scene, a landscape and a black cat in a coal bin?
Fast forward to 2008.
Camera manufacturers were quick to understand this exposure problem. Over the years as computer technology reached micro-miniaturization levels, the sophistication and "intelligence" of automated light metering evolved to provide new exposure options.
Center-weighted light metering
One of the first improvements (and still available on most modern DSLRs) was to devise an internal light metering system that concentrated its attention on the central part of an image. As the picture above illustrates, the light from the entire scene is measured but the reading from the center of the picture is given more WEIGHT in determining the final exposure. Why? Because most beginning and amateur photographers tend to place their most important compositional elements in the center of the image frame. This type of through the lens metering is called "center weighted". So camera manufacturers placed the greatest weight on the light levels in the center of the picture. This helps, but isn't a cure all. What if the object in the center of the viewfinder is a white cat? The exposure will tend to be too dark as the meter places first priority on making the white cat in the center of the picture an 18% gray cat.
In "Spot Metering" the camera bases its exposure recommendations only on the light being reflected from the area covered by the spot (center above)
The next addition was significant to a large number of professional photographers, but not always understood by the beginners and amateurs. The pros knew that if they FIRST measured the 18% gray object in the scene for the correct exposure and THEN composed their picture, they would have a well exposed and very printable image. The trick was remembering what to "point" the spot at when in "spot metering" mode.
The latest achievement, called matrix metering, (Nikon's term) takes the next logical step. Cameras equipped with a variant of matrix metering attempt to identify the scene by looking at a thousand or more points within the viewfinder image to determine "what" the scene is. (These systems read not black and white tone, but complete RGB color.) How? These cameras take the live red, green and blue metering information (and locations) and compare it to an on-board database of typical scenes -- in milliseconds. When it finds a close match, the camera recommends an f/stop and shutter speed (based on its scene/exposure library) that will render the scene as a correct exposure. Now that's smart. Like everything else, it's not perfect and can be fooled. But for the typical beginner, the chances of a correctly exposed picture have increased dramatically.
Of course, you need to spend some quality time with your camera's manual to determine the types of metering options available. The manual will also explain the markings seen through the viewfinder that designate exactly where the meter is taking its readings. Hopefully, this discussion will provide some guidance as to which metering mode is most appropriate for your shooting situation.
What does Hub use? For most of my shooting, I prefer the "spot metering" mode. Before I compose my picture, I determine what object should be about 18% gray (or 18% any color) in the final picture and meter off that object. My camera allows me to lock in that exposure setting. THEN I compose the picture, and go "click". The best bet is to:
- understand what and where the meter is measuring,
- the limitations of each mode,
- and experiment for yourself.
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