The invention discloses a fixed-focus optical
system which successively comprises seven lenses from the object side to the image side, wherein the first lens is provided with
negative power, a objectside convex surface and an image side convex surface; the second lens is provided with
negative power, a object side convex surface and an image side convex surface; the third lens is provided with
positive power and double convex surfaces; the image side surface of the fourth lens is a convex surface, when the focal power is positive, C4o is greater than or equal to 0, and the C4o is the curvature of the object side face of the fourth lens; when the focal power is negative, the object side face of the fourth lens is a
concave surface; the fifth lens is provided with
positive power and doubleconvex surfaces; the sixth lens is provided with negative
powder and two concave surfaces; the seventh lens is provided with
positive power and double convex surfaces; and the relationship is met: Imeg / f is greater than 1.28 but less than 2.22, and Imeg / TTL is greater than 0.105 but less than 0.167, wherein Imeg is the highest half image height, TTL is the
axis distance from the object side face of the first lens to the image side face, and f is
system focal distance. The
system has the advantages that the power is reasonably matched; by means of reasonable parameter matching, a miniature optical system design which is high in resolution, good in
image quality and low in cost is achieved.