However, the exposure instrument in the camera has a defect, that is, the metering area is too large, which leads to inaccurate metering. Even metering, metering area will account for more than 3% of the picture, which will lead to the failure to correctly expose the scene you want to expose. When shooting portraits, the luminosity of faces is very strict. Usually, the exposure meter behind the mirror can't meet the photometric accuracy. So the photographer chose to hold the event exposure instrument. This kind of exposure meter is simple to use, accurate in degree and widely used in artistic photography. This is very useful for the correct exposure of the work.