Imaging optical system, image capturing apparatus and electronic device

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

• a first lens element with positive refractive power having an object-side surface being convex in a paraxial region thereof;
• a second lens element having negative refractive power;
• a third lens element having an object-side surface being concave in a paraxial region thereof, both of the object-side surface and an image-side surface thereof being aspheric;
• and
• a fourth lens element with negative 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, both of the object-side surface and the image-side surface thereof being aspheric;
• wherein the imaging optical system has a total of four lens elements, and an air gap is arranged between every two adjacent lens elements;
• wherein the imaging optical system comprises an aperture stop disposed between an imaged object and the second lens element;
• and
• wherein a focal length of the second lens element is f2, a focal length of the fourth lens element is f4, a focal length of the imaging optical system is f, a maximum image height of the imaging optical system is ImgH, an axial distance between the second lens element and the third lens element is T23, an axial distance between the third lens element and the fourth lens element is T34, and the following conditions are satisfied:
• 1.0<f2/f4;
• 2.0<f/ImgH;
• and
• 1.0<T23/T34.