Patent ID: 8605368

Claim:
An image capturing optical system comprising, in order from an object side to an image side: a first lens element with positive refractive power having a convex object-side surface; a second lens element with negative refractive power having a concave object-side surface and a convex image-side surface; a third lens element with refractive power, wherein an object-side surface and an image-side surface of the third lens element are aspheric; a fourth lens element with positive refractive power, wherein an object-side surface and an image-side surface of the fourth lens element are aspheric; and a fifth lens element with refractive power having a concave image-side surface, wherein an object-side surface and the image-side surface of the fifth lens element which is made of plastic material are aspheric, and the fifth lens element has at least one inflection point formed on at least one of the object-side surface and the image-side surface thereof; wherein a curvature radius of the object-side surface of the second lens element is R3, a curvature radius of the image-side surface of the second lens element is R4, a focal length of the image capturing optical system is f, an axial distance between the object-side surface of the first lens element and the image-side surface of the fifth lens element is Td, an air conversion distance between the image-side surface of the fifth lens element and an image plane is BFL, a focal length of the fourth lens element is f4, and the following relationships are satisfied: 0< R 3/ R 4<1.0; 4 mm 2 <( Td×f )+( BFL×f )<10 mm 2 ; and 0< f/f 4<2.2.