Patent ID: 7499641

Claim:
A method for calculating an actual exposure time in a still image shooting operation of an optical apparatus which includes a mechanical diaphragm unit, a mechanical shutter, and an image sensor, the method comprising: calculating an acceleration of the shutter based on a time required for the shutter to shift between two points in a shutter closing action, and obtaining a relationship between a shutter shifting amount and time based on the calculated acceleration; obtaining, based on the relationship between a shutter shifting amount and time, a relationship between an aperture area and time, wherein the aperture area represents an area of an aperture defined by the diaphragm unit having a predetermined F-number and the shutter; obtaining a total light exposure during a period of time required for the shutter to completely close the aperture of the diaphragm unit having the predetermined F-number, by temporally integrating the aperture area; and obtaining an actual exposure time by dividing the total light exposure by a light exposure per unit time calculated from an aperture area of the diaphragm unit having the predetermined F-number in an opened state.