Patent ID: 7271965

Claim:
An apochromatic lens system for use with an external aperture stop, comprising: a lens barrel having an axis, a front end for facing an outside object, and a back end for focusing an image of the object onto an imaging surface aligned with the back end of the barrel; a first optical group having positive optical power; a second optical group having negative optical power; a third optical group having positive optical power; the first, the second, and the third optical groups are mounted in the lens barrel axially in the stated order from the front end to the back end of the barrel; the first optical group comprises four optical elements having, in order from the front end of the lens barrel, a negative optical power, a positive optical power, a negative optical power, and a positive optical power, and the first optical group is arranged to converge light received from an object at the front end of the barrel and to direct the converged light onto the second optical group; the second optical group comprises one optical element having a negative optical power and is arranged to diverge the light from the first optical group and to direct the diverged light onto the third optical group; and the third optical group comprises two optical elements each having a positive optical power, and the third optical group is arranged to converge the light from the second optical group and to focus the converged light onto the imaging surface; wherein, F′/F′ I >0.7; | F′ I /F′ II |>1.5; 0.35 <|F′ II /F′ III |<0.85; 0.45 <V 1 /V 2 ≈V 5 /V 6 ≈V 5 /V 7 <0.75; 0.85<( V 1 /V 2 )/( V 3 /V 4 )<1.15; 3.75 <IΔP 1 /ΔP 2 1 ≈ΔP 5 /ΔP 6 <4.75; 0.2<(Δ P 1 /ΔP 2 )/(Δ P 3 /ΔP 4 )<0.4; 0.85<(Δ P 3 /ΔP 4 )/(Δ P 5 /ΔP 7 )<1.15; and F′ is focal length of the lens system; F′ I , F′ II , and F′ III are focal lengths of the first, the second and the third optical groups; V 1 , V 2 , V 3 and V 4 are Abbe numbers for each of the optical elements of the first optical group in order from the front end of the lens barrel; V 5 is the Abbe number of the optical element of the second optical group; V 6 and V 7 are Abbe numbers for the optical elements of the third optical group in the stated order from the front end; and ΔP 1 , ΔP 2 , ΔP 3 , ΔP 4 , ΔP 5 , ΔP 6 and ΔP 7 are deviations of relative partial dispersion from linearity with respect to the Abbe number for the corresponding optical element; and the optical elements of the first, the second, and the third optical group are formed from at most three different glass materials.