Plastic lens assembly for use in copying machines

A four-element plastic lens assembly for use in a copying machine, comprising, in order from an object side, a first biconvex lens, a second biconcave lens, an aperture stop, a third biconcave lens which is substantially identical to said second lens, and a fourth biconvex lens which is substantially identical to said first lens, said lens assembly being substantially completely symmetrical with respect to said aperture stop, said first and fourth lenses being made of polycarbonate, and said second and third lenses being made of polymethyl methacrylate, said lens assembly meeting the following conditions:

• ##EQU5## where fs:
• the composite focal length of the entire lens assembly at reference temperature and humidity;
• f1s:
• the focal length of the first and fourth lenses at reference temperature and humidity;
• and
• f2s:
• the focal length of the second and third lenses at reference temperature and humidity;
• wherein assuming that r.sub.i :
• the radius of curvature of a refracting surface, d.sub.i :
• the distance between adjacent refracting surfaces, n.sub.i :
• the refractive index of a lens material, w:
• half angle of view, FNo:
• F number, and Y:
• the height of an object, said lens assembly has the following numerical values at a temperature of 20.degree. C. and a humidity of 65%:
• ______________________________________ fs = 99.990 mm FNo = 16 w = 22.degree. f1s = 24.202 mm f2s = -26.152 mm f1s/fs = 0.242, f2s/fs = -0.262 ______________________________________ r.sub.1 = 20.957 d.sub.1 = 1.574 n.sub.1 = 1.58972 r.sub.2 = -43.498 d.sub.2 = 0.145 r.sub.3 = -34.956 d.sub.3 = 1.539 n.sub.2 = 1.49394 r.sub.4 = 20.787 d.sub.4 = 3.480 r.sub.5 = -20.787 d.sub.5 = 1.539 n.sub.3 = 1.49394 r.sub.6 = 34.956 d.sub.6 = 0.145 r.sub.7 = 43.498 d.sub.7 = 1.574 n.sub.4 = 1.58972 r.sub.8 = -20.957. ______________________________________