The Neutral Gray Standard  

All light meters, regardless of the type, are designed to measure light in a consistent way. Light meters presume all subjects are of average reflectance, or a neutral gray — often called “middle” gray because it falls in the middle of the zones between pure black and pure white. In the Zone system of exposure, this middle gray is known as Zone V.
The use of the neutral gray standard allows a reflected light meter to render correct readings for “average” subjects in “average” lighting situations. Light meters, however, can’t see subjects and interpret them the way you can — they measure only one thing: the intensity of light. Fine if you’re photographing a medium gray man in a medium gray suit on an average day — but not entirely accurate in other situations.

Using Reflected Measurement  
  
Hand-held reflected light meters (including built-in camera meters) read the intensity of light reflecting off the subject. Because they measure the light after it hits the subjects, however, they are affected by the reflectance of the subject’s surfaces. Also, because most reflected readings are taken from the camera position they generally take in a wide area that can include many different reflective surfaces or colors that can bias the meter reading. If you’re photographing a person walking on a sandy beach on a bright day, for instance, the light reflecting off of the sand will overwhelm the reading and result in an underexposed image of the person.
A reflected meter will provide different readings for say, a white cat and a black cat — but it will provide an exposure that records both as the same middle gray. Similarly, a pristine fresh-fallen snow and a black coal field will be recorded as the same color: medium gray. A reflective meter will also record a red apple and a green apple as the same tone — even though in reality they reflect vastly different amounts of light. You can improve the accuracy of your reflected readings by placing an 18% neutral gray test card in front of the important subject areas — but that’s not always practical.

 
Advantages Of Incident Meters  
 
A better alternative to reading the light in many scenes is to use an “incident” meter. Hand-held incident meters read the intensity of light falling on the subject and are usually taken from the subject position. Because they are not affected by variances in subject color or reflectance, incident meters accurately record the amount of light falling onto the subject. In the majority of situations, an incident reading is extremely accurate and records tones, colors and values correctly.

 

Advantages of Reflected Spot Measurements   
 
Some reflected meters, known as “spot” meters, have the facility to take reflected readings of tiny areas of subject from the camera position. Spot metering is an ideal solution in a lot of special situations where incident readings are impossible and normal reflected readings are unreliable, including: measuring distant subjects, backlight, extreme brightness range, highly reflective surfaces, or when the main subject may be moving.
Spot metering of an 18% gray midtone area allow you to make an exposure that will record detail, tonality and color accurately. Once you get accustomed to “reading” scenes, you’ll be able to tell which subject areas have an “average” reflectance and even in the most contrasty situations, you’ll be able to take spot readings from critical areas. By taking a spot reading of a tall green Saguaro cactus (a medium-toned subject) in the middle of a bright sandy desert, for instance you’ll get an accurate reading of the important subject area. Spot meters also allow you to average readings from several brightness areas within a scene.

In-Camera Spot Meters Vs. Hand-Held Meters   
  
Although a number of advanced SLRs offer spot-metering capability, the metering angle is directly related to the focal length of the camera lens in use. Every time the lens is changed, the effective spot-meter angle changes. With a normal lens in use, the spot-metering angle may be 15-degrees or more. A hand-held 1-degree spot meter, on the other hand, allows the most selective measurement of distant subjects, as well as small areas in more complex scenes.