Optical imaging system, image capturing unit and electronic device

An optical imaging system comprising, in order from an object side to an image side:

• a first lens element having an object-side surface being convex in a paraxial region thereof;
• a second lens element with positive refractive power having an image-side surface being convex in a paraxial region thereof, wherein at least one of an object-side surface and the image-side surface of the second lens element is aspheric;
• a third lens element with negative refractive power having an object-side surface being concave in a paraxial region thereof, wherein at least one of the object-side surface and an image-side surface of the third lens element is aspheric;
• a fourth lens element with positive refractive power having an object-side surface being concave in a paraxial region thereof and an image-side surface being convex in a paraxial region thereof, wherein at least one of the object-side surface and the image-side surface of the fourth lens element is aspheric;
• and
• a fifth lens element with negative refractive power having an image-side surface being concave in a paraxial region thereof, wherein the image-side surface of the fifth lens element has at least one convex shape in an off-axis region thereof, and both of an object-side surface and the image-side surface of the fifth lens element are aspheric;
• wherein the optical imaging system has a total of five lens elements, the fourth lens element has the largest central thickness among the first lens element, the second lens element, the third lens element, the fourth lens element and the fifth lens element;
• wherein an axial distance between the first lens element and the second lens element is T12, a central thickness of the second lens element is CT2, a central thickness of the fourth lens element is CT4, a central thickness of the fifth lens element is CT5, a curvature radius of the object-side surface of the third lens element is R5, a curvature radius of the image-side surface of the third lens element is R6, a focal length of the first lens element is f1, a focal length of the second lens element is f2, and the following conditions are satisfied:
• 1.05<T12/CT2;
• 1.30<CT4/CT5;
• (R5+R6)/(R5−R6)<−0.25;
• and
• |f2/f1|<0.80.