Patent ID: 7224498

Claim:
An image reading apparatus, comprising: illumination means for illuminating an original; reading means for reading image information; and an imaging optical system for imaging information provided on a surface of the original, as illuminated by said illumination means, upon a surface of said reading means, said imaging optical system including, in order from the original surface side, a first lens group of negative refractive power, a second lens group of positive refractive power, and a third lens group of negative refractive power; wherein the first lens group is held fixed for magnification change, and a condition 0.01<R×K/F<1.00 is satisfied where R is resolution per single line in a main scan direction of said image reading apparatus, K is picture element size of said reading means, and F is F number of said imaging optical system at a largest imaging magnification, wherein a focal point position can be adjusted by moving the second lens group, and wherein a condition 0.04>Lt/F 2 >0.10 is satisfied where F 2 is the focal length of the second lens group, and Lt is one of (i) the spacing between the first and second lens groups at a telephoto end and (ii) the spacing between the second and third lens group at a telephoto end, which is narrower than the other.