1. Field of the Invention
The present invention relates to an imaging lens, the longitudinal chromatic aberration of which has been corrected especially from a visible light range to a near-infrared range, and which is appropriate for use in monitoring both day and night, and the like. Further, the present invention relates to an imaging apparatus on which the imaging lens has been mounted.
2. Description of the Related Art
Conventionally, a Gauss lens, which is a symmetric optical system, and a modified Gauss lens were used as fixed focus lenses of medium telephoto class. For example, U.S. Pat. No. 5,640,277 (Patent Document 1) discloses an invention related to a medium telephoto photography lens using a front group part of a Gauss lens, as the first lens group of the photography lens. The photography lens disclosed in Patent Document 1 adopts an inner-focus-type focusing method, in which an inner lens group is a movable group. Besides the photography lens disclosed in Patent Document 1, U.S. Pat. No. 7,466,490 (Patent Documents 2) discloses a fixed focus lens adopting an inner-focus-type focusing method in an objective optical system for an endoscope.
Since lenses for monitoring and the like are often used both day and night (the same lens is used for day and night), it is necessary that longitudinal chromatic aberration is corrected from the visible light range to the near-infrared range. Further, as network cameras became widely used, and the resolution of sensors (imaging devices) became higher in recent years, a demand for imaging lenses that can cope with a large pixel number of 5 megapixels or the like is increasing also for monitoring use. Therefore, it is necessary to suppress longitudinal chromatic aberration for a wide wavelength range in an excellent manner. Meanwhile, as described above, an optical system that is called as the Gauss lens, and which is a symmetric optical system, was often used as a fixed focus lens of medium telephoto class from the past. However, it was difficult to correct longitudinal chromatic aberration for a wide wavelength range by using the Gauss lens. Further, in the Gauss lens, variation of aberrations depending on photography distances was relatively large.