The exit pupil is a concept in optics, and is defined as the minimum diameter of the light beam leaving an eyepiece though which all of the light from the eyepiece passes. For a telescope, it 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 divided by the magnification. See also list of optical topics. ... 50 cm refracting telescope at Nice Observatory. ... The focal point F and focal length f of a positive lens, a negative lens, a concave mirror, and a convex mirror. ... An eyepiece is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. ... A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ...

Ideally, the exit pupil should be about the size of the observer's pupil when the eye is positioned by the eyepiece; then, the maximum amount of light transmitted through the system reaches the retina, and the image is not vignetted. Human eye cross-sectional view. ... A vignette is a small decorative design, or a brief descriptive piece of writing. ...

For example, a set of 7 x 50 binoculars has an exit pupil a trifle 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 os only 4mm 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 reaching the retina. 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. Binoculars A set of binoculars (from Latin, bi-, two-, and oculus, eye) is a hand-held tool used to make distant objects appear closer by passing the image through two adjacent series of lenses, and erecting prisms. ...