An optical apparatus includes a mechanical shutter in addition to a mechanical diaphragm. The optical apparatus obtains a shutter shifting amount based on acceleration data of the shutter, obtains a total light exposure in a period of time required for the shutter to completely close an aperture of the diaphragm having a predetermined F-number, and calculates 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 having the predetermined F-number in an opened state.