Optical system, camera module and electronic device

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

• a first lens having a positive refractive power;
• a second lens having a refractive power;
• a third lens having a refractive power;
• and
• a fourth lens having a refractive power, an image side surface of the fourth lens being concave on an optical axis, an object side surface and the image side surface of the fourth lens being aspherical, and at least one of the object side surface and the image side surface of the fourth lens having an inflection point;
• wherein the optical system further comprises a stop arranged between an object side of the first lens to the fourth lens;
• and
• the optical system satisfies the conditions:
• 0.5<SL/TTL<0.9,
• −1<SAG31/CT3<0,
• 20 mm<OBL<40 mm, and
• FNO≤3.55;
• wherein SL is a distance from the stop to an imaging surface of the optical system on the optical axis, TTL is a distance from an object side surface of the first lens to the imaging surface of the optical system on the optical axis, and SAG31 is a vector height of an object side surface of the third lens, and CT3 is a thickness of the third lens on the optical axis;
• wherein OBL is a distance from a subject to be imaged by the optical system to the object side surface of the first lens on the optical axis;
• and
• wherein FNO is an f-number of the optical system.