Metering

• Within cameras, light is measured as the amount of light that is reflected from an object into the viewfinder of the camera. Cameras measure reflected light against an average color of middle gray, judged in printing terms as reflecting 18 percent of light. In practice, all objects do not reflect light evenly, meaning that an exposure can be over or underexposed depending on the light reflected from the object. Spot metering allows the photographer to measure the light reflected by a small area of the object anywhere in the viewfinder. Partial metering allows the photographer to measure the light reflected from an object in the center of the frame.
  
  

Frame

• As an automatic feature of modern cameras, the use of a partial metering mode allows the photographer to measure the reflected light of the subject in the central 13.5 percent of the viewfinder. Spot metering requires the photographer to find an area of the viewfinder that reflects an average color area. In subjects with difficult colors for spot metering, a specially designed gray card can be used to gain an accurate meter reading that will stop any difficulties with under or overexposure.
  
  

Use

• Partial metering techniques are generally used for portrait photography and in situations where the main subject of the portrait is backlit and appears in silhouette. Partial metering techniques help photographers judge the exposure of the frame from the colors of the subject and not the background lighting. Partial metering techniques commonly provide an underexposed subject and an overexposed background. Spot metering in contrast is used for difficult lighting conditions such as interior photographs and photos requiring a small subject to be exposed perfectly.
  
  

Area

• Disadvantages of using spot metering are that some cameras have a large spot metering area that can cause problems with gaining an accurate measurement of the area to be metered. In other cameras with a smaller spot metering area, the exposures gained are more accurate than a larger metering area. As partial metering judges the light reflected in the central portion of the frame, the light metered is not always accurate.