 The reading from the light meter is combined with aperture settings, exposure times, and film or sensor sensitivity to calculate the optimal exposure.Light meters typically average the light in a scene to 18% middle gray.More advanced cameras are more nuanced in their metering, weighing the center of the frame more heavily (center-weighted metering), considering the differences in light across the image (matrix metering), or allowing the photographer to take a light reading at a specific point within the image (spot metering).