🔭 Eyepiece & Magnification Calculator

Work out what an eyepiece actually gives you on a given telescope: magnification, exit pupil, true field of view and the practical limits. Covers Barlows and finds your maximum useful power.

Enter your telescope and eyepiece.
Formulas and rules of thumb

Magnification = telescope focal length ÷ (eyepiece focal length × Barlow). True field = apparent field ÷ magnification. Exit pupil = aperture ÷ magnification = effective eyepiece focal length ÷ focal ratio.
Below about 0.5 mm exit pupil the image dims and diffraction softens; above your own pupil diameter you throw light away. Maximum useful power is roughly 2× aperture in millimetres (50× per inch), and only on steady nights.