As has been mentioned many times by various posters, the camera meter measures reflected light and assumes that, averaged out, the scene should be at 18% grey and sets the exposure accordingly. This means that if you take a picture of a black cat (and it takes up most of the image), the camera will adjust the exposure so that the cat will be 18% grey (ie, overexposed). If you shot a white sheet of paper, the camera would underexpose getting the same 18% grey value.
However, if you meter the incident light, then white things stay white, black things stay black, and so on. Since the sun's light output is pretty constant (and life on spaceship Earth would be very difficult if it wasn't), the issue is whether the light that falls on a subject is diminished (by shade, cloud, haze, etc.) or increased (reflected light added to existing light). In the days before lightmeters were common, people would set their exposure according to the guidlines on the film box. One convenient rule was the Sunny 16 rule, which said that on sunny days (ie, strong shadows), shooting at f/16, you set the shutter speed closest to the ISO rating. Say you were using ISO 100 film, you'd want to shoot at a shutter speed of 1/125 (125 is closest to 100). Shooting on a bright sunny day and on sand or snow, you get additional light reflected, so increase the shutter speed by 1 stop (ie, 1/250). Going the other way, sunny with weak shadows is 1 stop more (1/60), bright cloudy (but no shadows) would be another stop (1/30), and open shade is another stop (1/15). By the way, read Fred Parker's "Ultimate Exposure Computer" article
http://www.thephotoforum.com/forum/...4579-please-help-light-meter-questions-2.html. He discusses this in much better detail, and gives a table showing exposure values which allow you to know (without the lightmeter) what exposure a typical scene will need.
In the Zone system, you have eleven zones, with zone 0 being totally black, and zone X being totally white (
DPanswers: Digital Zone System Tutorial is one good explanation). Excluding the end zones, you have eight zones with useful detail, with zone V being the midpoint (18% grey). By coincidence, my digital camera has a dynamic range of about 8 as well, so in my situation one stop up or down is equivalent to one zone up or down. The zone system tries to set the exposure (and film processing, but that's not relevent for us with digital cameras) so that as much of the target scene will fit into the relatively limited range that the camera can capture. (By way of comparision the average human eye has a dynamic range of 15 or more). Therefore by adjusting the exposure (manually), you can truly control the exposure so that most things in the scene are well exposed, with useful detail.
Practically, if you have confidence in your light meter, you put your camera on spot-meter mode and read the exposure values from the darkest and brightest parts of the scene where you want detail, and if the difference in values is eight stops or more, then you just can't squeeze the scene into the camera without losing some highlights or dark detail. Then you have to decide which is more important to you, and bias your exposure accordingly.
If you don't have a reliable light-meter, use the guidelines for manual exposure, and the chances are pretty good that you'll have good exposures.
