Achromatic objective lens

Claim 1

An achromatic objective lens comprising: positive, negative and positive lenses, a first positive lens L 1 , a negative lens L 2 and a second positive lens L 3 , satisfying the following conditions: (1) 40<ν 2 <45 (2) P 2 * <0.5700, P 2 ** >0.3300 (3) ν 1 >ν 2 +15 (4) |ν 3 -ν 2 |<5 (5) P 3 * -P 2 * >0.005, P 2 ** -P 3 ** >0 (6) 0.15 f<-f 2 <0.5 f (7) 0.5<f 1 /f 3 <2.0 where: ν i is the Abbe's number of the lens L i , P i * is the partial dispersion ratio of short wavelength part of the lens L i , where: ##EQU3## and n g , n C and n F are the refractive indexes corresponding to g-line (436 nm), C-line (656 nm) and F-line (486 nm) of the optical glass material, respectively, P i ** is the partial dispersion ratio of long wavelength part of the L i , where: ##EQU4## and n A' , n C and n F are the refractive indexes corresponding to A'line (768 nm), C-line and F-line of the optical glass material, respectively, f is the overall focal length, and f i is the focal length of the lens L i , and the lens system further defined as follows: ______________________________________ f = 100.0 F No. 1 : 12 Lens Surface Distance along optical axis ______________________________________ R.sub.1 = 55.2096 L.sub.1 d.sub.1 = 1.000 R.sub.2 = -28.8000 d.sub.2 = 0.049 R.sub.3 = -28.5730 L.sub.2 d.sub.3 = 0.417 R.sub.4 = 23.5163 d.sub.4 = 0.097 R.sub.5 = 22.6201 L.sub.3 d.sub.5 = 1.000 R.sub.6 = -79.7936 ______________________________________ n.sub.d ν P* P** ______________________________________ L.sub.1 1.51633 64.15 0.5350 0.3582 L.sub.2 1.61340 43.84 0.5623 0.3373 L.sub.3 1.53256 45.91 0.5747 0.3312 P.sub.3 * - P.sub.2 * = 0.0124 P.sub.2 ** - P.sub.3 ** = 0.0061 -f.sub.2 = 20.966 f.sub.1 /f.sub.3 = 1.108, and n.sub.d is the refractive index on the d-line. ______________________________________