The Meter

In order to capture an image, the sensor must receive the proper amount of light.  Too much or too little light and the image may not be recoverable.  The in-camera light meter measures the light from the scene being shot either through the lens, or through a separate aperture.  The other type of meter is an external meter or "Hand Held Meter".  The hand held meter is a much more accurate way to measure exposure because it measures the light from the light source, unlike the in-camera meter which measures reflected light. 

In-camera light meters measure the light reflected from the scene.  One type of reflected type meter is the center weighted average meter.  This type of meter assumes that what you are taking a picture of, 

Center Weighted  Meter showing the area in white that is being metered

is in the center of the frame, and that an "average" amount of light is being reflected from what's being photographed.  The average amount of light is calculated to be 14%, and most scenery reflects approximately 14% of the light that fall on it.  Discrepancies in reflected light can usually be adjusted post process in photoshop, lightroom, or any number of photo enhancing programs.  The problem with any reflected light meter is it assumes that there is 14% of light being reflected.  What if you took a picture in the snow, or on a black lava bed?  In the case of the snow, the camera would assume the snow is  14% gray, and underexpose the image, and it would assume the black lava is %14 gray, and overexpose the image.  As a photographer, you must understand and recognize when the meter is being fooled by the amount of reflected light.

Another type of in-camera meter is the spot-meter.  The spot meter also measures reflected light, but in a very small spot in the center of the viewfinder.  The function of the spot meter is to allow a more precise area to be measured, for instance, if you were taking a picture of a car, you could point the spot at the car, depress the shutter button partway down to set the meter, and then recompose shot and finally take the photo. The spot meter can also

Spot Meter showing the area in white being metered

allow you to analyze the lighting from different parts of the scene, and allow you to average the lighting on your own.  These two types of meters dominated the photo industry for many years until the late 80's, and the age of the computer.  One of the problems posed by meters are, they aren't smart.  They have no way of knowing what you are trying to accomplish when taking a picture, so a small percentage of the time, you'll end up with a photo with an improper exposure, even though the meter told you otherwise.

Introducing the Matrix Meter, and the smart camera.  You thought Artificial Intelligence was only in the future, but A.I. has been around for a long time.  The Matrix Meter uses not one, but many meters to try and assess what you are trying to photograph, and calculate the best exposure for that image.

   The meters are arranged in a pattern or matrix in the hopes that your photograph fits one of several types of photographs that people normally take.  In the example on the left, the image is a landscape of a lake with a distant shoreline.  One of the programs inside the camera will assess this and determine the proper exposure.  This type of metering is far more accurate than the two previously mentioned, but like all reflected light meters, it's only as smart as it's circuitry and programmer.

The last meter is the pinnacle of accuracy, but it is separate from the camera.  The hand held light meter is not a reflected meter, but an incident light meter.  This means that the light illuminating the scene is measured, not the light that is being reflected, so it doesn't matter if you are photographing white sand or black lava, the exposure will be the same.  The light is measured in an arc 180 degrees, using a small white dome on the meter.  The dome is pointed at the camera as near to the object being photographed as possible.  The proper exposure is read on a dial or from an LCD screen.  Some handheld light meters will also measure both natural light and light from a flash, or both at the same time.

On a budget and can't afford an expensive hand held light meter, but you still want the accuracy?  You can buy a gray card that is calibrated for 14% gray.  all you need to do is place it in the same light that is falling on the subject and use in-camera meter to judge the proper settings for the gray card.  Since 14% of the light is reflected, the exposure will be accurate for the scene falling under the same light.  A gray card can also help you set the proper white balance in the image.  White balance has to do with "Color Temperature."  Color temperature is measured in Kelvins or K for short.  If you took a piece of iron and placed it in the dark and then heated it, it would begin to glow red, then orange, yellow, white and finally blue before disappearing into ultraviolet.  At a faint red glow the iron would be 770 degrees Kelvin, a photo flood lamp is around 2700 degrees.  A Halogen lamp is around 3200 degrees, while a photo flash or the sun is around 5600 degrees Kelvin.   Finally, anything in the shade during the daytime is illuminated by the sky which is about 9000K to 12000K.  Your eyes have an automatic white balance as well so no matter what type of lighting we are under, it all seems white, but back in

Portable folding gray card

the days of film, you had to compensate for white balance especially if you used slide film.  If you tried to take photographs with indoor lighting, everything came out orange.  If you bought indoor film and shot outdoors, everything turned out blue.  To compensate you had to use filters on the lens.  Digital cameras have the ability to white balance automatically, however, just like the lightmeter getting fooled, the white balance meter can get fooled too, for instance, if you were photographing something that was predominantly blue, or orange, the camera would try to adjust the photo to make it appear neutral.  A gray card is balance neutral gray, so if you use a gray card to set your white balance, your photo will come out perfect.  You can also fix the photo after the fact if you include the gray card in your first photo, and use photoshop or similar program to white balance the photo.