Everything we have discussed thus far and much of what we will cover depends upon you using your meter as accurately as possible. With this is mind you should understand what your type of meter is doing and how to use it most effectively.
There are several manufacturers of hand held meters and all modern cameras have in-camera metering systems. There are those that are digital and those that are manual. But our concern is how the various types of meters read light and what information they are really giving.
Many terms are used to describe a meter: averaging, spot, incident, reflective, multi - weighted, strobe, in-camera,etc. These can come in almost any combination. You can have a simple reflective spot meter or an in-camera averaging meter or a hand held, incident strobe meter w/ spot metering the point being that you can find whatever combination to fit your needs. But its been my observation that many people buy more meter than they ever use. for example -If your primary interest is landscape photography, you have little use for a strobe setting, etc.
Your concerns when purchasing a meter should be: 1 - What type of photography are you interested in ( e.g. - if you are doing studio work, a strobe meter is a must), 2 - How accurate do you wish to be with your exposures and 3 - what is the exact information your type of meter is giving you.
All meters can be divided into two basic categories:
REFLECTIVE - These meters read the amount of light that is reflecting off the subject or object. They take that information and translate it as a middle gray (typically referred to as 18 percent gray) regardless of how bright or dark the subject. A white wall in the sun or a black dress in a dark closet, they are the same shade of gray as far as a reflective meter is concerned. The reflective value of these objects are always translated to an exposure reading that will interpret them as a middle gray value or Zone V.
There are two types of reflective meters :
Spot - reads a small area of the scene as small as 1 or 2 %. The advantage of using this type of meter is that you can very accurately and precisely read small areas of a scene to determine your exposure and the scene's contrast range.
Averaging - reads a larger area of the scene and gives you an average of all the values that it covers. This type of meter can sometimes cause problems depending on where in the scene you are pointing the meter. If you are pointing it into the shadow area of the scene, you can often overexpose the highlights. If you point it into the brighter area of the scene, you can often underexpose the entire scene. You have to keep in mind that it is averaging the area it is reading.
Incedent - These meters are designed to read the amount of light that is falling on the subject, rather than the light reflecting off the subject. But what information are they really giving you? The information they are giving you is the same reading you would get if you held a middle gray card in the same spot you are holding the meter and used a reflective meter to obtain the reading. In other words, an incident meter tells you what the correct exposure would be to properly translate that middle gray card in that area of the subject. All other objects within the total subject would fall in place in relationship to that middle gray value, in that area of the photograph.
One of the problems you run into when using these meters is not considering this fact. for example, if you were photographing someone in a white dress in partial sun and your reading was ƒ/ 5.6 @ 1/ 125 Sec. on the shadow side of a subject's face (using an incident meter), but if a reflective reading taken off the dress in the sun was ƒ/ 32 @ 1/ 125 Sec., then the dress would be overexposed by two or three stops if the incident shadow reading were used to expose the negative. Or if you took a reading from both the shadow and sun side and split the difference (as most do and is recommended), you would more than likely be loosing some shadow detail and overexposing the highlight.
This is not to say that these are not very effective meters. They are the meters of choice of most professional photographers, especially in studio situations where strobe is being used as the primary light source.