Telescope Exit Pupil Calculator

The exit pupil is the diameter of the cone of light leaving the eyepiece and entering your eye, measured in millimeters. It equals the telescope aperture divided by the magnification, or equivalently the eyepiece focal length divided by the focal ratio. Matching it to your eye's pupil affects image brightness and comfort.

Exit pupil = aperture diameter / magnification, where magnification = telescope focal length / eyepiece focal length. Combining the two gives exit pupil = eyepiece focal length / focal ratio, and focal ratio = telescope focal length / aperture. All lengths use millimeters so the exit pupil comes out in millimeters.

For deep-sky views you typically want a large exit pupil (4 to 7 mm) to maximize brightness, but no larger than your eye's dark-adapted pupil, which shrinks with age from about 7 mm to 5 mm. For planetary and double-star work a small exit pupil (0.5 to 1 mm) at high magnification gives the sharpest detail.

Surface brightness of an extended object scales with the square of the exit pupil. A larger exit pupil delivers a brighter image, up to the point where it exceeds your eye's pupil, beyond which the extra light is wasted on your iris and the effective aperture is reduced. The optimal exit pupil therefore matches your dilated pupil.

Enter the telescope aperture, telescope focal length, and eyepiece focal length all in millimeters. The calculator returns the magnification (dimensionless), the focal ratio (the f-number), and the exit pupil in millimeters so you can compare it against your eye's pupil size.