The present invention, a first lens having a positive refractive power; a second lens having a negative refractive power; a third lens having positive refractive power; a fourth lens having a negative refractive power; a fifth lens having positive refractive power; a sixth lens having negative refractive power, wherein the first lens, the second lens, the third lens, the fourth lens, the fifth lens, and the sixth lens are sequentially arranged from the object side to the image side, the conditional expression 0.6 < HFOV A compact telephoto optical system satisfying / TTL < 1.4 (HFOV: horizontal angle of view of optical system, TTL: length from the object side of the first lens to the image surface of the image sensor) is provided. According to the present invention, it is possible to implement a small telephoto optical system having an angle of view of 30 degrees horizontally and 33.3 degrees diagonally. In addition, by applying a planar or near-planar shape to some lens surfaces, it is possible to improve assembly properties and improve sensitivity to assembly tolerances. In addition, constant resolution performance can be realized regardless of temperature change.
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