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A camera lens adjusted for large and small aperture. The entrance pupil is the image of the physical aperture, as seen through the front of the lens. The size and location may differ from those of the physical aperture, due to magnification by the lens.

The apparent location of the anatomical pupil of a human eye (black circle) is the eye's entrance pupil. The outside world appears to be seen from the point at the center of the entrance pupil. (The pupil itself, which lens designers would call the aperture, is in a slightly different location because the image is magnified by the cornea.)

入射光瞳（英語：entrance pupil）也叫入射瞳、入瞳，是指 In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil. If there is no lens in front of the aperture (as in a pinhole camera), the entrance pupil's location and size are identical to those of the aperture. Optical elements in front of the aperture will produce a magnified or minified image that is displaced from the location of the physical aperture. The entrance pupil is usually a virtual image: it lies behind the first optical surface of the system.

The geometric location of the entrance pupil is the vertex of the camera's angle of view and consequently its center of perspective, perspective point, view point, projection centre or no-parallax point. This point is important in panoramic photography, because the camera must be rotated around it in order to avoid parallax errors in the final, stitched panorama. Depending on the lens design, the entrance pupil location on the optical axis may be behind, within or in front of the lens system; and even at infinite distance from the lens in the case of telecentric systems.

In photography, the size of the entrance pupil (rather than the size of the physical aperture itself) is used to calibrate the opening and closing of the diaphragm aperture. The f-number ("relative aperture"), N, is defined by N = f/E_N, where f is the focal length and E_N is the diameter of the entrance pupil. Increasing the focal length of a lens (i.e., zooming in) will usually cause the f-number to increase, and the entrance pupil location to move further back along the optical axis.

The entrance pupil of the human eye, which is not quite the same as the physical pupil, is typically about 4 mm in diameter. It can range from 2 mm (f/8.3) in a very brightly lit place to 8 mm (f/2.1) in the dark.

参见

• 出射光瞳
• Transmittance
• Pupil magnification

外部链接

• Stops and Pupils in Field Guide to Geometrical Optics Greivenkamp, John E, 2004