How it works
Magnification is the telescope's focal length divided by the eyepiece's focal length. A 1,000 mm telescope with a 10 mm eyepiece gives 100×. A Barlow lens multiplies that number.
True field of view is roughly the eyepiece's apparent field divided by the magnification. The full Moon is about half a degree wide, so a field above 0.5° shows the whole disk.
Exit pupil is the width of the beam of light leaving the eyepiece. Around 4–7 mm suits wide views of star clusters and nebulae; 1–2 mm suits the Moon and planets. Above about 7 mm some light is wasted because a dark-adapted eye cannot open wider.
The highest useful magnification is about 2× per millimetre of aperture (50× per inch), and only on steady nights. Resolution uses the Dawes limit and faintest stars use a common rule of thumb, so both are estimates.
Everything is calculated in your browser. Your location is used only on this page and is never sent to us.
Magnification and eyepiecesHow to work out magnification, its real limits, and what eyepieces change.
Types of telescopesRefractors, reflectors and catadioptrics: how each works and which suits whom.
Choosing your first telescopeWhat really matters, what to avoid, and what you will actually see.
How to use a telescope: your first nightSet up, cool down, align the finder and focus: a calm first night with a telescope.