The present disclosure provides an optical photographing lens system comprising four lens elements, the four lens elements being, 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; a third lens element; and a fourth lens element with positive refractive power having an object-side surface being convex in a paraxial region thereof and an image-side surface being concave in a paraxial region thereof, the image-side surface of the fourth lens element having at least one convex critical point in an off-axial region thereof, both the object-side and the image-side surfaces thereof being aspheric.