The invention relates to an optical system which, albeit being simple in construction, enables images of a wide range of natural subjects to be well reproduced with their colors, and provides an image pickup system comprising, at least, an image pickup optical system 1, an electronic image pickup device 3 having three or more different spectral characteristics to obtain a color image and a controller 4 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 in said image pickup system comprises 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 700-nm wavelength at which an output signal strength ratio with respect to an input quantity of light is highest with the proviso that 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 4 in response to the light beam.