What I love unique to Nikon D90 is its Scene Recognition System incorporating a 420-pixel RGB CCD that references against a database of over 30,000 shooting situations (not images) to propose the ideal exposure setting when you squeeze that trigger. Yes, it's ramped up in the bigger brothers like the Nikon D700 to 1005-pixel. The three metering mode uses this engine in a slightly different manner
- Matrix - A system that divides the whole images into pieces and analyzes the difference between them. In addition, other input consideration factor like autofocus information, overall scene brightness, distance and focal length information from lens. For example, if Dynamic Area Autofocus is used, it tends to look at the pattern of the sensor (11 points) to determine how much to bias the exposure
- Center-weighted - measures the entire frame, but effectively separates it into two zones, the central area and the outer area. The exposure is based 75% on the central area, 25% on the outer area. The central measuring area is normally 8mm (customizable to 6 or 10mm) This mode is recommended for potraits.
- Spot - Most professionals tend to use spot metering when they have enough time to do a critical evaluation of a scene