Image pickup apparatus capable of taking color natural images and high-resolution images of a monochrome object

An image pickup apparatus which is capable of forming an image signal having a high resolution when a monotone image is photographed by a structure having a switching circuit for switching between a first photographing mode in which an electric signal transmitted from an image pickup device is used to form a color natural image signal and a second photographing mode for forming a high resolution image signal of a monochrome object to insert/remove an optical low pass filter in accordance with the switching operation, wherein the length of the optical passage is corrected by another optical system.