An image forming lens for forming an image of an original document includes a front side lens group arranged at an object side and a rear side lens group arranged at an image forming side. The front side lens group includes from 2 to 4 lenses including at least one positive lens, and the rear side lens group includes one negative lens. The back-focus of the image forming lens when in use is equal to or smaller than 25% of an entire length of the image forming lens, and the open space between the front side lens group and the rear side lens group is equal to or greater than 50% of the entire length of the image forming lens. The front side lens group lens can include two, three or four pieces of lenses. By configuring an image forming lens with at least three lenses, correction of aberration is easy and high performance can be realized.

In a zoom lens of two-unit construction comprising a first lens unit having negative refractive power as a whole and a second lens unit having positive refractive power as a whole, the first lens unit comprises, in succession from the object side, a negative lens having its concave surface of sharp curvature facing the image side and a positive lens having its convex surface facing the object side, the second lens unit has, in succession from the object side, at least a positive lens, a positive lens, a negative lens having its concave surface of sharp curvature facing the image side, and a positive lens having its convex surface of sharp curvature facing the image side, the first lens unit has at least one aspherical surface, and the zoom lens satisfies the condition that 0.65<.vertline.f.sub.1 /(f.sub.w .multidot.f.sub.T).sup.1/2 .vertline..ltoreq.1.1.

Disclosed is an image reading apparatus which includes an illumination system, a reading unit, and an imaging optical system for imaging imagewise information provided on a surface of an original as illuminated by the illumination system, upon a surface of the reading unit. The imaging optical system includes, in an 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. The image reading apparatus has a focal point position adjusting function and a focal length changing function based on movement of at least one of the lens groups of the imaging optical system. In the imaging optical system, a condition 0.01<R.times.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 the reading unit, and F is F number of the imaging optical system at a largest imaging magnification.

Variable focal length lens system comprises two adjacent lens units, one stationary, the other movable. These lens units are in order from an object side towards an image side (i) A first lens unit possessing divergent refractive power. The first lens unit is the front most lens unit. It consists of a single negative lens component. (ii) A second lens unit possessing convergent refractive power. The second lens unit, includes at least two positive power lens components and an aperture stop.

An imaging device includes a refractive imager lying in an optical path, and optionally a telescope that directs the optical path to the refractive imager. The refractive imager includes a first lens group that forms an intermediate image of a scene on the optical path, wherein the first lens group includes a first-lens-group positive-power lens, and a first-lens-group negative-power lens. A second lens group relays the intermediate image to a final image surface on the optical path, wherein the second lens group includes a second-lens-group positive-power lens, and a second-lens-group negative-power lens. A third lens group may be selectively inserted into the optical path between the first lens group and the second lens group and selectively removed from the optical path. The third lens group includes a third-lens-group positive-power lens, and a third-lens-group negative-power lens.