Patent ID: 8704937

Claim:
An imaging optical system comprising, in the order from an object side toward an image side: a first lens element having a positive refractive power as a whole; a second lens element having a negative refractive power as a whole, a surface position at a maximum effective diameter of the second lens element being located on the object side than a surface vertex of the second lens element; a third lens element having a positive refractive power as a whole; and a fourth lens element having a negative refractive power as a whole, the fourth lens element having an aspherical surface in a contour of a cross section taken along and through an optical axis, the aspherical surface having an inflection point in a direction from the optical axis to an end of an effective area, wherein the imaging optical system satisfies the following conditional expressions: 0.7 <f 1 /f< 5 (1) −0.8<( RS 1 +RS 2)/( RS 1 −RS 2)/( RS−RS 2)<3 (2) −3<( RS 3 +RS 4)/( RS 3 −RS 4)<2 (3) 0.03 <d 2 /TL< 0.2 (4) 2 W> 72 (5) ν4>50 (6) 0.55 <Y/TL< 0.8 (7) where f1: a focal length of the first lens element, f: a focal length of an entirety of the optical system, RS1: a curvature radius of an object-side surface of the first lens element, RS2: a curvature radius of an image-side surface of the first lens element, RS3: a curvature radius of an object-side surface of the second lens element, RS4: a curvature radius of an image-side surface of the second lens element, d2: an optical axis distance between the first lens element and the second lens element, TL: a total length of the entirety of the optical system (in the case where a parallel plate is disposed, the length of the parallel plate is calculated, assuming that the parallel plate is air), W: a maximum half angle of view, ν4: an Abbe number of the fourth lens element, and Y: a maximum image height.