Patent ID: 8891142

Claim:
An image reading lens which reads an original image, comprising: a front group lens system on an object side; and a back group lens system on an image side, the front group lens system including three lenses having two positive lenses and a negative lens, and the back group lens system including a plastic negative lens, wherein the following conditional expressions are satisfied where Σdn N /dt(N) is a sum of values 10 6 times a refractive index temperature coefficient for an e-line in the negative lens of the front group lens system at 20° C. to 40° C., Σdn p /dt(P) is a sum of values 10 6 times a refractive index temperature coefficient for an e-line in the positive lens of the front group lens system at 20° C. to 40° C., fa is a focal length of the front group lens system, f is a focal length of an entire image reading lens, d 9 is a distance on an optical axis between the front group lens system and the back group lens system, and D is an entire length of a lens, 0 <Σdn N /dt ( N )−Σ dn P /dt ( P )<17 1 0.20 <fa/f×d 9 /D< 0.90. 2