An image forming apparatus for producing a video signal, comprising: an optical system configured to form an optical image of an object; an imaging element configured to image the optical image and output image data representing an array of pixels in a sequence of frames; and an exposure control unit configured to control the imaging element and comprising: a first unit configured to determine an optimum exposure amount variation (updata) based on a difference between a sum of luminance values of pixels of a first frame output by the imaging element and a predetermined target value; a second unit configured to determine a corrected optimum exposure amount variation (updata') by multiplying the optimum exposure amount variation (updata) by a value derived from dividing a current exposure amount of the first frame (exposure) by a maximum exposure amount (exposure max) obtainable by the imaging element; a third unit configured to determine an optimum exposure amount (exposure') by adding the corrected optimum exposure amount variation (updata') to the current exposure amount (exposure); a fourth unit configured to determine an exposure time according to the optimum exposure amount (exposure'); and a fifth unit configured to set the exposure time in the imaging element for a second frame subsequent to the first frame. an optical system configured to form an optical image of an object; an imaging element configured to image the optical image and output image data representing an array of pixels in a sequence of frames; and an exposure control unit configured to control the imaging element and comprising: a first unit configured to determine an optimum exposure amount variation (updata) based on a difference between a sum of luminance values of pixels of a first frame output by the imaging element and a predetermined target value; a second unit configured to determine a corrected optimum exposure amount variation (updata') by multiplying the optimum exposure amount variation (updata) by a value derived from dividing a current exposure amount of the first frame (exposure) by a maximum exposure amount (exposure max) obtainable by the imaging element; a third unit configured to determine an optimum exposure amount (exposure') by adding the corrected optimum exposure amount variation (updata') to the current exposure amount (exposure); a fourth unit configured to determine an exposure time according to the optimum exposure amount (exposure'); and a fifth unit configured to set the exposure time in the imaging element for a second frame subsequent to the first frame. a first unit configured to determine an optimum exposure amount variation (updata) based on a difference between a sum of luminance values of pixels of a first frame output by the imaging element and a predetermined target value; a second unit configured to determine a corrected optimum exposure amount variation (updata') by multiplying the optimum exposure amount variation (updata) by a value derived from dividing a current exposure amount of the first frame (exposure) by a maximum exposure amount (exposure max) obtainable by the imaging element; a third unit configured to determine an optimum exposure amount (exposure') by adding the corrected optimum exposure amount variation (updata') to the current exposure amount (exposure); a fourth unit configured to determine an exposure time according to the optimum exposure amount (exposure'); and a fifth unit configured to set the exposure time in the imaging element for a second frame subsequent to the first frame.