On Exposure
 
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 (18%) 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.
   
Incident Metering
Reflected Metering
   
Because incident metering reads the intensity of light falling on the subject, it provides readings that will create accurate and consistent rendition of the subject’s tonality, color and contrasts regardless of reflectance, background color or brightness or subject textures. Subjects that appear lighter than middle gray to your eye will appear lighter in the finished image. Subjects that are darker than middle gray will appear darker. Colors will be rendered accurately and highlight and shadow areas will fall naturally into place. Neat trick, eh? Because reflected metering reads the intensity of light reflecting off of the subject, they are easily fooled by variances in tonality, color, contrast, background brightness, surface textures and shape. What you see is often not at all what you get. Reflected meters do a good job of reading the amount of light bouncing off of a subject — the trouble is they don’t take into account any other factors in the scene. They are merciless in recording all things as a medium tone.
 

Reflected measurements of any single tone area, for instance, will result in a neutral gray rendition of that object aka 18% Gray.

A light subjects ie. a white cat that appear lighter than gray will reflect excess light and cause them to record darker than they appear ie. an 18 % gray cat instead of a white one. To get a pure white cat one would have to ADD exposure which can be done in a manner of ways:

1. dial in between +1/3 to +1 in the exposure compensation dial or
2. decrease shutter speed to allow more light in (slower speed than metered)
3. increase aperture to allow more light in (bigger aperture than metered)

A dark subject ie. like a black cat that are darker than gray will reflect less light and result in an exposure that renders it lighter — in other words, an 18 % gray cat instead of a black one. To get a pure black cat, one would have REDUCE exposure which can be done in a manner of ways:

1. dial in between -1/3 to -1 in the exposure compensation dial or
2. increase shutter speed to less more light in (faster speed than metered)
3. decrease aperture to allow less light in (smaller aperture than metered)

Or to ensure that the meter is measuring 18 % Gray as 18 % Gray, carry along an 18 % Gray card that meter off that instead to ensure accurate exposure measurement before tripping the shutter. This would be a route recommended for all cameras that use TTL metering (most if not all current SLRs).
   
Incident measurements on old Rolleiflex/Rolleicords or similar cameras can be had by taking the exposure measurements from the subject's location while pointing camera to the intended taking position ensuring the same light falling on the subject is also now falling on the camera.
 
White Plate
   
Gray Plate
   
Black Plate
   
The Sekonic L-358 Flash Master is an advanced, yet easy to operate, exposure analyzing light meter, that incorporates the latest technology in handheld meters.

www.sekonic.com

Courtesy of Sekonic at http://www.sekonic.com/IncidentVsReflect.html
 
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