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Telescope field of view calculator

True FOV = Apparent FOV ÷ Magnification

How the calculation works

Magnification first: telescope focal length divided by eyepiece focal length. Then divide the eyepiece's apparent field of view by that magnification to get the true field of view — the patch of sky you actually see.

Typical apparent FOVs

  • Plössl eyepiece: 50–52°
  • Wide-field designs (Erfle, Panoptic): 65–70°
  • Ultra-wide (Nagler, Ethos): 82–100°

What this tells you

A 1200 mm telescope with a 25 mm Plössl gives 48× magnification and a true FOV of about 1.08° — enough to fit the Pleiades cluster (M45) almost completely. Swap to a 10 mm eyepiece and the field shrinks to 0.43°, which is roughly the size of the Moon's disc.