Optical system comprising graded refractive index lens element

An optical system comprising, in the order from the object side:

• at least a first lens unit having a positive refractive power;
• and
• a second lens unit having a negative refractive power with an airspace between said first and second lens units, said second lens unit and said first lens unit disposed next to one another so that no other lens components are disposed between the first and second lens units;
• wherein said optical system varies focal length thereof by varying the airspace reserved between the first and the second lens unit;
• and
• wherein said first lens unit includes at least one graded refractive index lens element satisfying the following conditions (1) and (4):
• n(r)=n.sub.0 +.SIGMA.n.sub.1 r.sub.1.sup.21 ( 1)
• .PHI..sub.S .multidot..PHI..sub.M <0 (4)
• wherein the reference symbol n.sub.0 represents the refractive index of said graded refractive index lens element as measured on an optical axis thereof, the reference symbol r designates distance as measured in a radial direction from the optical axis, the reference symbol n(r) denotes the refractive index of said graded refractive index lens element as measured at a distance r from the optical axis, the reference symbols n.sub.1, n.sub.2, n.sub.3, . . . represent refractive index distribution coefficients of the second, fourth, sixth, . . . orders respectively, the reference symbol .PHI..sub.S designates the refractive power of a surface of said graded refractive index lens element, and the reference symbol .PHI..sub.M denotes the refractive power of a medium of said graded refractive index lens element and is expressed as .PHI..sub.M =-2n.sub.1 d.sub.G, where d.sub.G is the thickness of said graded refractive lens element as measured on the optical axis.