In a quantity-of-light adjusting apparatus having an iris and an ND filter which limit incident light, a control method for the quantity-of-light adjusting apparatus, and a computer program product providing a control program for the quantity-of-light adjusting apparatus, a state of limitation of the incident light by the iris is changed at a first changing speed, and, when a state of limitation of the incident light by the ND filter is changed, the state of limitation of the incident light by the iris is changed at a second changing speed different from the first changing speed.