| CPC G02B 13/0045 (2013.01) [G02B 9/62 (2013.01); G02B 13/006 (2013.01)] | 18 Claims |

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1. An optical system, sequentially arranged from an object side to an image side, comprising:
a first lens having a positive refractive power;
a second lens having a refractive power;
a third lens having a negative refractive power, an object side surface and an image side surface of the third lens being both concave at circumferences thereof;
a fourth lens having a negative refractive power;
a fifth lens having a positive refractive power, at least one of an object side surface and an image side surface of the fifth lens having an inflection point; and
a sixth lens having a negative refractive power;
wherein the optical system satisfies the following relational expressions:
0.60<CT1/SD11<1.01;
5.5<TTL/CT1≤6.84; and
2.2≤FNO≤2.6;
wherein, CT1 is a thickness of the first lens on an optical axis, SD11 is half of a maximum effective aperture of an object side surface of the first lens, TTL is a distance from the object side surface of the first lens to an imaging plane of the optical system on the optical axis, and FNO is an f-number of the optical system.
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