Imaging optical system and imaging apparatus

An imaging optical system includes at least one lens, and an optical element having a transmittance that changes in a radial direction perpendicular to an optical axis. The transmittance of the optical element is minimized at a first position in the radial direction separated from an optical axis position of the optical element by a distance smaller than an effective radius rd of the optical element. The transmittance at the first position is smaller than that at the optical axis position and that at a second position separated from the optical axis position by rd.