1. Field of the Invention
The present invention relates to an optical system, and an optical instrument, an image pickup apparatus, and an image pickup system using the same.
2. Description of the Related Art
In a case of observing a minute sample, a method in which, first, the overall sample is observed, and a region to be observed in detail is identified, and thereafter the region to be observed in detail is magnified and observed, has hitherto been adopted. As an image pickup apparatus to be used in such method, an image pickup apparatus which magnifies digitally an image that has been captured, and displays the magnified image is available. As an optical system to be used in such image pickup apparatus, an optical system described in Japanese Patent Application Laid-open Publication number 2012-173491 is available. Digital magnification of image is called as digital zooming.
Moreover, if conventional optical systems, such as optical systems for microscope, are differentiated according to a difference of a type of image formation, they will be divided into two types namely, optical systems of finite correction type and optical systems of infinite correction type. In the optical system of finite correction type, an object image is formed at a finite distance by a microscope objective. Whereas, in the optical system of infinite correction type, light emerged from the microscope objective becomes a substantially parallel light beam. Therefore, in the optical system of infinite correction type, an object image is formed by combining the microscope objective and a tube lens.
As aforementioned, in a microscope optical system of the infinite correction type, a microscope objective by which, the light emerged becomes substantially parallel light beam, has been used. As an example of the microscope objective, a microscope objective described in Japanese Patent Application Laid-open Publication No. 2008-185965 is available. The microscope objective described in Japanese Patent Application Laid-open Publication No. 2008-185965 has a numerical aperture (NA) of an extremely large value on an object side (sample side), such that a numerical aperture on the object side is 0.8. This microscope objective is used with the tube lens, and at this time, if a numerical aperture on an image side is small, a bright and sharp image cannot be formed.