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Magnification and field-of-view calculator

Enter your telescope and eyepiece to see the magnification, how much sky fits in the view, and what will look good.

Start from a common setup

130 mm reflector80 mm refractor200 mm Dobsonian
The diameter of the main lens or mirror
Often printed as “f=650mm”
The number on the eyepiece, such as 25 mm
Usually 50–52° for standard eyepieces

Result

Magnification26×Within range
True field2.00°3.8 × the full Moon
Useful maximum260×Twice the aperture
Exit pupil5.0 mmComfortable

Wide and bright. Great for the Pleiades, the Orion Nebula and sweeping the Milky Way. Saturn’s rings will be small but visible.

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How it works

Magnification is the telescope’s focal length divided by the eyepiece’s focal length: a 650 mm telescope with a 25 mm eyepiece gives 26×. True field of view, the patch of sky you actually see, is approximately the eyepiece’s apparent field divided by the magnification. The full Moon is about half a degree across, which is why the result compares the field with it.

The useful maximum magnification is about twice the aperture in millimeters; beyond it the image gets bigger but not sharper. On most nights the atmosphere limits any telescope to roughly 150–250×. Exit pupil is the aperture divided by the magnification: around 5–7 mm gives the brightest, widest views for faint objects, 1–2 mm suits planets, and below about 0.5 mm the view becomes too dim to be useful.

  • Swap eyepieces to change magnification; the telescope’s focal length stays fixed.
  • A 2× Barlow lens doubles the telescope’s effective focal length, and so doubles the magnification.
  • Not sure what to start with? Read binoculars or telescope.