An optical system enables images to be well reproduced with their colors, and provides an image pickup system including an image pickup optical system, an electronic image pickup device to obtain a color image and a controller for implementing signal processing or image processing on the basis of an output from the electronic image pickup device. The optical element that takes part in the determination of a focal length includes an optical element making use of a refraction phenomenon alone. The 400-nm wavelength input/output ratio is 10% or less with respect to an input-output ratio for a 400-nm to 800-nm wavelength at which an output signal strength ratio with respect to an input quantity of light is highest when the input quantity of light is defined by the quantity of a light beam emanating from the same object point and entering the image pickup optical system and the output signal strength is defined by the strength of a signal produced from the controller in response to the light beam.