This invention provides an optical lens system in order from an object side to an image side comprising: a first lens element having a convex object-side surface; a negative second lens element having a concave object-side surface and a convex image-side surface; a positive third lens element having a convex object-side surface and a convex image-side surface; a plastic negative fourth lens element having a concave object-side surface and a convex image-side surface, with both the object-side and image-side surfaces thereof being aspheric. By such arrangement, the incident angle of off-axis light projected onto the sensor can be suppressed for improving the sensitivity of the sensor effectively. Also, a sufficient back focal length can be retained for disposing other optical elements (e.g., an IR-pass filter), and thereby the system can be more suitable for the infrared aspect of optical imaging systems.