Fundamentals of Lens Optics 4 : Pupil of the Optical System

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  • レンズ光学の基礎４：光学系の瞳

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Abstract

The pupil of a lens is an image of the aperture stop. The entrance pupil of a lens or lens system is the image of the aperture stop as seen from the object space. The exit pupil, however, is the image of the aperture stop as seen from the image space. In other words, the entrance pupil, the aperture stop and the exit pupil are conjugate planes.  In combining a microscope objective and a relay lens, the aperture stop of the microscope objective and that of the relay lens must be conjugate planes. It is important that the exit pupil of the microscope objective and the entrance pupil of the relay lens are conjugate planes. If not so, the image intensity will suddenly decrease at the edge of the field. As a solution to this, a field lens is inserted between the objective and the relay lens. For the same reason it is desirable that the pupil of the image sensor is coincident with the exit pupil of a lens system.  Spherical aberration of the pupil causes formation of a shadow at the edge of the field. The shadow moves about in the field as the eye is moved. Longitudinal chromatic aberration of the pupil causes image color difference between the field center and field edge. The image color in some parts of the field changes as the eye is moved.

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