| 摘要 |
PURPOSE: To decrease the fluctuations of the optical characteristics of lens unit according to a temp. change and to well correct secondary spectra over the entire variable power range so as to maintain high optical performance by adequately setting the lens materials of the lens units. CONSTITUTION: This zoom lens is composed of the plural lens units including a focusing unit (first group) F. A compensator (third group) C has positive lenses G9, G12, G14 consisting of anomalous dispersion glass for decreasing the secondary spectra and a negative lens G13 consisting of general heavy flint glass. The materials of the positive lenses G9, G12, G14 consist essentially of fluorine or boron added with P2 O5 and satisfy the conditions 62<νp , Np <1.62, 0.605-(νp /1000)<Qp when their refractive index is defined as Np , their Abbe number asνp and their partial dispersion ratio asθp . The material of the negative lens G13 is added with SiO2 and TiO2 respectively at a weight ratio of >=20% and satisfies the conditionsνN<30, 1.75<NN when its refractive index is defined as NN and its Abbe number asνN.
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