Optical imaging system comprising four lenses of - -+-, +-+-, +++- or -++- refractive powers, image capturing unit and electronic device

An optical imaging system comprising four lens elements, the four lens elements being, in order from an object side to an image side, a first lens element, a second lens element, a third lens element and a fourth lens element;

• each of the four lens elements having an object-side surface facing toward the object side and an image-side surface facing toward the image side;
• wherein the image-side surface of the first lens element is convex in a paraxial region thereof, the fourth lens element has negative refractive power, an absolute value of a focal length of the first lens element is larger than an absolute value of a focal length of the third lens element, an axial distance between the first lens element and the second lens element is larger than a central thickness of the second lens element, and an absolute value of a curvature radius of the image-side surface of the second lens element is larger than an absolute value of a curvature radius of the image-side surface of the first lens element;
• wherein a focal length of the optical imaging system is f, a focal length of the fourth lens element is f4, 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 central thickness of the third lens element is CT3, a central thickness of the fourth lens element is CT4, and the following conditions are satisfied:
• −0.44≤f/f4<0;
• 0.99≤(R5+R6)/(R5−R6)<3.0;
• and
• CT4<CT3.