The invention is based on a method for controlling a flash unit in connection with a digital image sensor without a mechanical shutter, wherein the exposure of the image sensor is performed in sub-areas, such as pixel rows or columns, or in another order, by using a rolling electronic shutter or the like. Thus, the exposure of said sub-areas of the image sensor takes place at least partly at different times. According to the invention, during the time of exposure of one image, i.e. substantially all the sub-areas of the image sensor, several discrete, successive flashes of the flash unit are used to achieve even exposure of the image area, which the flashes are timed in such a way that each single flash is triggered at a point of time which is common to the integration or exposure time of as many successive sub-areas of the image sensor as possible. According to the invention, the successive light flashes are also timed in such a way that one and only one flash of the flash unit occurs during the integration or exposure time of each single sub-area. By the present invention, the significant advantage is attained that the invention minimizes the number of light flashes required for one image while it also secures that each sub-area of the sensor to be exposed at a different time is illuminated as evenly as possible.