Meters used to take reflected readings are pointed at the subject and the exposure is calculated from the light reflected from the subject. There can be a large variation in calculated exposure if the reading is not taken properly. It is essential to understand what the meter is reading. Simply, the meter is manufactured to read the amount of light reflected from a scene and then calculate a combination of aperture and shutter speed to correctly expose the film so that the area read by the meter will reproduce as a 'mid tone of grey'. The photographer must evaluate the subject, decide on an area that is near this 'mid tone of grey' and take the reading from this part of the subject. If for instance the meter is pointed at the subject in a generalized way, large areas of bright or dark tones can influence the meter reading falsely, resulting in over or under exposure.