Exit pupil
From Wikipedia, the free encyclopedia
In optics, the exit pupil is a virtual aperture in an optical system. Only rays which pass through this virtual aperture can exit the system. The exit pupil is the image of the aperture stop in the optics that follow it. The same term is used to refer to both the virtual aperture and its diameter.
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[edit] Calculation
For a telescope, the diameter of the exit pupil can be calculated by dividing the focal length of the eyepiece by the focal ratio (f-number) of the telescope. More simply, it is the diameter of the objective lens divided by the magnification.
[edit] Image brightness and clarity
In a system designed for direct visual observation, the exit pupil should be about the size of the pupil of the observer's eye, and it should be placed a comfortable distance past the eyepiece, so the observer can place his or her pupil in the plane of the exit pupil. This allows the maximum amount of light to reach the retina, and the image is not vignetted.
For example, a set of 7 x 50 binoculars has an exit pupil just over 7 mm, the average human eye pupil size at night. The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of brightness at night due to using such binoculars (assuming perfect transmission). In daylight, when the pupil is only 4 mm in diameter, over half the light will not reach the retina. However, the loss of light in the daytime is generally not significant since there is so much light to start with. By contrast, 8 x 32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. That's just enough to fill a typical daytime eye pupil, making these binoculars better suited to daytime use than night-time.
[edit] In digital photography
The concept of exit pupil is not often discussed in books on photographic optics, but has become a very important topic for digital cameras. The distance of the exit pupil from the focal plane determines the range of angles of incidence that the focal plane will see, and image sensors, especially with microlenses, often have a limited range of angles over which they will efficiently accept light. The closer the exit pupil to the focal plane, the higher the angles of incidence at the extreme edges of the field.
An online "digital lens FAQ" article explains the importance of exit pupil distance in defining the concept of a lens suitable for a digital camera [1].
