Ben Horne's tutorial on use of manual spot metering in digital cameras
here which is a simplified version of the zone system:
meter on concrete, light gray stonework, bright afternoon sky or fair skin then set so exposure is +2 stops
meter on average blue sky w/o polariser, Caucasian skin tones then set exposure to + 1 stop
meter on darker vegetation, lush green grass or deep blue sky opposite to sun then set exposure to neutral.
the zone system created by Ansell Adams which consists of 9 zones each 1 f-stop difference in light intensity
in B&W film use, they would often determine correct exposure to get shadow details and then adjust development to alter the film contrast to retain highlight details. So they might meter on a shadow detail area in which they want to appear dark in the print but still retain good detail (ie. zone III) and then reduce the exposure by 2 stops from what the meter indicated (otherwise it will make this region zone V). To then get the contrast range, point spot meter at the brightest highlights in which you want details and then to determine what zone this is, determine how many stops difference from your original meter of zone III, then add 3 to it. If this falls in zone VIII, then can develop as for normal development.
in transparency film or digital use, it is usually more important to avoid over-exposure and blowing out the highlights, so it is better to expose for highlight details and let the shadows take care of themselves.
sunny days often given contrast range of 9 f-stops which is too much for digital capture - see
dynamic rangeespecially of use with spot meters
thus you will note that if you use your camera meter to expose on Caucasian skin, your image will come out 1 stop under-exposed.
to get a properly exposed sunset, meter the area directly above the sun (without including the sun). If you want the scene to look like it's a half-hour later, stop down by one f-stop, or set exposure compensation to minus one.