1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus including the same, which is suitable for use in a television camera for broadcasting, a video camera, a digital still camera, and a silver-halide camera.
2. Description of the Related Art
In recent years, there have been demanded a zoom lens system having a large diameter ratio, a high zoom ratio, and high optical performance for image pickup apparatuses such as a television camera, a silver-halide camera, a digital camera, and a video camera.
A positive lead and telephoto type four-unit zoom lens system in which four lens units are provided in total and one of the lens units located closest to an object side has a positive refractive power has been known as the zoom lens system having a high zoom ratio. For example, U.S. Pat. No. 7,304,805 and Japanese Patent Application Laid-Open No. 2005-084409 disclose a four-unit zoom lens system which includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a negative refractive power, and a fourth lens unit having a positive refractive power.
U.S. Pat. No. 7,304,805 and Japanese Patent Application Laid-Open No. 2005-084409 disclose a four-unit zoom lens system in which an optical material having extraordinary dispersion is used to excellently correct a chromatic aberration, to thereby obtain high optical performance.
When the optical material having extraordinary dispersion is used for the four-unit zoom lens system as described above, a lateral chromatic aberration and an axial chromatic aberration are excellently and easily corrected. In order to obtain high optical performance, it is important to excellently correct a lateral chromatic aberration at a wide angle end and an axial chromatic aberration at a telephoto end.
For example, in order to correct the lateral chromatic aberration at the wide angle end, the optical material having extraordinary dispersion only needs to be used for a lens which is located in an image plane side of an aperture stop and has a positive power (refractive power). However, in a case of an optical system having a relatively small F-number, the lens has no small effect on an on-axial beam, and hence the axial chromatic aberration tends to be overcorrected (axial chromatic aberration is excessively corrected).
When the optical material having extraordinary dispersion is used to correct the lateral chromatic aberration at the wide angle end, the axial chromatic aberration tends to deteriorate in the entire zoom range. In order to increase the optical performance of the four-unit zoom lens system as described above over the entire zoom range, it is important to suitably set a lens structure of the fourth lens unit which does not move for zooming.