A focus extending optical system has optical lenses and a focus extender. The optical lenses form an image of light, from an object, on an image sensor. The focus extender adjusts a wavefront so as to change a position of the image, formed by the optical lenses, in accordance with a distance from an optical axis and thereby extends a focus range. The focus extending optical system satisfies a condition that a value of a second MTF is less than or equal to three times a value of a first MTF. The first MTF is an MTF at a spatial frequency of ½ of a Nyquist frequency of the image sensor. The second MTF is an MTF at a spatial frequency of ¼ of the Nyquist frequency.