PURPOSE: To obtain the image forming lens for reading an image, which is short in overall length of the lens system, miniature and inexpensive and satisfactory in the quality of a formed image by providing a first and a fourth biconvex lenses and a second and a third biconcave lenses, and allowing them to satisfy specific conditions.
CONSTITUTION: This lens is provided with a first lens L1 consisting of a biconvex lens, a second and a third lenses L2, L3 consisting of a biconcave lens, respectively and a fourth lens L4 consisting of a biconvex lens in order from an object side, and constituted so as to satisfy conditional expressions shown by 0.35<f1/f<0.51, 0.34<f4/f<0.45, 0.11<d7/f<0.16, and 0.29<d/f<0.41. In this case, (f), f1, f4, (d) and d7 denote a composite focal length of the whole system, a focal length of a first lens L1, a focal length of a fourth lens L4, a distance on an optical axis between the face of the object side of a first lens L1 and the face of an image side of a fourth lens L4, and thickness on the optical axis of a fourth lens L4, respectively. Also, a diaphragm (i) is provided between a third lens L3 and a fourth lens L4.