Patent ID: 7319896

Claim:
A capsule endoscope comprising: a light source; an objective optical system that forms an image of an object that is illuminated by the light source; an image pickup device that receives the image formed by the objective optical system; and a transparent cover that is positioned in front of the objective optical system; wherein the following conditions are satisfied: cos(θ/2)< a 1 /b 1 <1.0 0.3≦{ f 2 (1−cos(θ/2))/ X B ·cos(θ/2)}/|Δ I|≦ 1.0 where θ/2 is the half-field angle, as measured from the optical axis of the objective optical system, that corresponds to the maximum image height of an object; a 1 is the air-equivalent distance along the optical axis of the objective optical system from the front focal point of the objective optical system to the exterior surface of the transparent cover in the on-axis direction; b 1 is the air-equivalent distance from the front focal point of the objective optical system to the exterior surface of the transparent cover in a direction that corresponds to the half-field angle θ/2, as measured from the optical axis; f is the focal length of the objective optical system; X B is the air-equivalent distance from the front focal point of the objective optical system to an object; and ΔI is the average value of the curvature of field of the tangential image surface T and the sagittal image surface S of the objective optical system at the half-field angle θ/2, where θ/2 is measured from the optical axis and the curvature of field is measured from the paraxial image plane.