Patent ID: 7828721

Claim:
An electronic endoscope comprising: an insertion unit that is inserted into a subject; a channel formed in the insertion unit and through which a treatment tool can be inserted; a single-focus objective optical system provided at a distal end of the insertion unit to form an optical image of the subject; and an image capturing element having a light receiving surface placed at a position where the objective optical system forms an image, to execute a photoelectric conversion on the optical image formed on the light receiving surface, wherein an image of a subject comprising a black band and a white band which have the same width is captured via the objective optical system and a brightness signal is generated from the resulting image signal and a maximum value of a brightness signal for the white subject is defined as Imax, a minimum value of a brightness signal for the black subject is defined as Imin, and a contrast I is defined by I=(Imax−Imin)/(Imax+Imin), and wherein the objective optical system and the channel are formed such that: an image signal is output such that the contrast I defined above is at least almost 10%, the contrast I being obtained by capturing an image of a subject comprising a 0.5-mm pitch black and white band pair at a first object distance of 50 mm from the distal end of the insertion unit, and at a second object distance shorter than the first object distance at which an image signal is output such that the contrast I defined as described above is at least almost 10%, the contrast I being obtained by capturing an image of a subject comprising a 35-μm pitch black and white band pair, an image of vicinity of the distal end of the treatment tool projected from a distal end opening in the channel is formed on the light receiving surface of the image capturing element.