Patent ID: 8837053

Claim:
A zoom lens comprising: an aperture diaphragm; and a plurality of lens units including a negative lens unit arranged on a light incident side of the aperture diaphragm, the zoom lens being configured to make variable a focal length by changing an interval among the plurality of lens units, wherein the negative lens unit includes a negative lens having a radial type refractive index distribution and a concave surface on the light incident side, and wherein the negative lens has a wavelength dispersion distribution in which a difference per unit thickness between a power for a F-line and a power for a C-line in a refractive index distribution of a gradient-index defined by the following expression is positive on an optical axis of the zoom lens, and the difference per unit thickness monotonously decreases as a distance from the optical axis increases: Δ∂φ N,FC =−2( N 10,F −N 10,C )−4( N 20,F −N 20,C ) r 2 −6( N 30,F −N 30,C ) r 4 −8( N 40,F −N 40,C ) r 6 , where a value Δ∂φ N,FC is the difference per unit thickness, r is the distance from the optical axis in a radial direction of the negative lens, N 10,F , N 20,F , N 30,F , and N 40,F are power series coefficients of the radial type refractive index distribution for the F-line given by the following expression, and N 10,C , N 20,C , N 30,C , and N 40,C are power series coefficients of the radial type refractive index distribution for the C-line given by the following expression: N F ( r )= N 00,F +N 10,F r 2 +N 20,F r 4 +N 30,F r 6 +N 40,F r 8 , and N C ( r )= N 00,C +N 10,C r 2 +N 20,C r 4 +N 30,C r 6 +N 40,C r 8 , where N 00,F is a refractive index on the optical axis of the negative lens for the F-line, N 00,C is a refractive index on the optical axis of the negative lens for the C-line, N F (r) is the radial type of refractive index distribution for the F-line, and N C (r) is the radial type of refractive index distribution for the C-line.