Spot metering

In photography, the metering mode refers to the way in which a camera determines the exposure.

Cameras generally allow the user to select between spot, center-weighted average, or multi-zone metering modes.

Various metering modes are provided to allow the user to select the most appropriate one for use in a variety of lighting conditions.

With spot metering, the camera will only measure a very small area of the scene (between 1-5% of the viewfinder area). This will by default be the very centre of the scene. The user can select a different off-centre spot, or to recompose by moving the camera after metering. The first spot meter was built by Arthur James Dalladay, editor of The British Journal of Photography in about 1935 and described it in the BJP Almanac of 1937 on pages 127 to 138.

A few models (including the Olympus OM-4, Canon T90 and in the digital world, the Olympus C-5050z) support a Multi-Spot mode which allows multiple spot meter readings to be taken of a scene that are averaged. Some cameras, the OM-4 and T90 included, also support metering of highlight and shadow areas.

Spot metering is very accurate and is not influenced by other areas in the frame. It is commonly used to shoot very high contrast scenes. For example, if the subject's back is being hit by the rising sun and the face is a lot darker than the bright halo around the subject's back and hairline (the subject is "backlit"), spot metering allows the photographer to measure the light bouncing off the subject's face and expose properly for that, instead of the much brighter light around the hairline. The area around the back and hairline will then become over-exposed. Spot metering is a method upon which the Zone System depends. In many cases the camera will over or underexpose, when using the spot mode. Modern cameras tend to find the correct exposure precisely. In complex light situations though, professional photographers tend to switch to manual mode, in order to ensure the same exposure in every photograph.

Another example of spot metering usage would be when photographing the moon. Due to the very dark nature of the scene, other metering methods tend to overexpose the moon. Spot metering will allow for more detail to be brought out in the moon while underexposing the rest of the scene. More commonly, spot metering is used in theatre photography, where the brightly lit actors stand before a dark or even black curtain or scrim. Spot metering only considers the actors in this case, while ignoring the overall darkness of the scene.

...
Wikipedia