Patent ID: 7505802

Claim:
A capsule endoscope comprising: a capsule having a central axis; a light source; an objective optical system that forms an image of an object illuminated by light emitted by the light source; an image pickup device that receives the image formed by the objective optical system; and a transparent cover having a central axis, said transparent cover being located in front of the objective optical system; wherein the objective optical system is arranged on the central axis of the transparent cover and the capsule; and the following conditions are satisfied 2 f 2 {1−cos(θmax/2)}/ D ·cos(θmax/2)+Δ I≦ 0 and 0.4 <IH max/ f where f is the composite focal length of the objective optical system; θmax/2 is the angle of view at the maximum image height, as measured between the optical axis of the objective optical system and a chief ray; D is the external diameter of the capsule endoscope; ΔI is the average curvature of field when a flat object, positioned at a distance D/2 from the object side of the objective optical system, is observed, with ΔI being defined as the average curvature of field of the sagittal image surface and the tangential image surface of the objective optical system at the maximum angle of view; and IHmax is the maximum image height on the paraxial image plane; and the following conditions are also satisfied within the range of θ/2=40° through θmax/2: a 0 >0 and, a p ≦0 where θ/2 is the half-angle of view, as measured from the optical axis of the objective optical system; a 0 is the distance from a hypothetical, hemispherical surface of radius of curvature R equal to D/2 to a near point of the depth of field on the axis, and the direction away from the optical system is regarded as positive; and, ap is the distance from a hypothetical, hemispherical, surface where R=D/2 to a near point of the depth of field off the axis and the direction away from the optical system is regarded as positive.