Telescope Magnification Calculator: Eyepiece, Exit Pupil, Field of View

Your details

The diameter of the primary mirror or lens. Also called the objective diameter.
mm
The focal length of the telescope, usually printed on the tube or in the manual.
mm
The focal length of the eyepiece you are using, printed on its barrel.
mm
The apparent (manufactured) field of view of your eyepiece. Typical Plossl: 50-52 deg. Wide-angle: 68-82 deg. Ultra-wide: 100+ deg.
deg
Multiplier from a Barlow lens (e.g. 2 for a 2x Barlow) or focal reducer (e.g. 0.63 for a 0.63x reducer). Leave at 1 if not using one.
x
A fully dark-adapted human eye opens to about 7 mm when young; this narrows to 5-6 mm after age 50. Used to calculate minimum useful magnification.
mm
MagnificationGood general use
40x

How many times larger the image appears compared to the naked eye

Exit pupil5mm
True field of view1.3deg
Resolving power0.58arcsec
Limiting magnitude13.5
Maximum useful magnification400x
Minimum useful magnification29x
f/ratio5
Surface brightness factor51%
5 mm
Too dim<0.7High power0.7-2.5General use2.5-5Wide-field5-7Excess pupil7+

40x magnification with a 5.00 mm exit pupil.

  • Exit pupil of 5.00 mm is within the useful range (0.7-7 mm), so all the light from the telescope reaches your eye.
  • 40x falls in the practical range (29x to 400x) for this telescope.
  • True field of view is 1.30 deg, roughly 2.6 full Moon diameters across.

Next stepFor deep-sky objects like galaxies and nebulae, aim for an exit pupil of 3-7 mm. For the Moon and planets, 1-2 mm is ideal.

 = Powered by OnlyCalculators