What is apparent field of view (AFOV)?

AFOV is the angular width of the image circle as seen through the eyepiece alone, independent of any telescope. It is stamped on the eyepiece barrel or listed in its specifications. Typical values range from 45° for basic Kellner designs to 100°+ for premium wide-field eyepieces.

How is true field of view (TFOV) calculated?

TFOV = AFOV ÷ Magnification, where Magnification = Telescope Focal Length ÷ Eyepiece Focal Length. For example, a 52° AFOV eyepiece at 40× magnification gives a 1.3° true field of view.

Why does TFOV matter for deep-sky observing?

Large deep-sky objects such as the Andromeda Galaxy (3° wide) or the Pleiades cluster (2°) only fit in a single eyepiece view when the TFOV is wide enough. Knowing your TFOV helps you choose the right eyepiece instead of guessing at the telescope.

What exit pupil should I aim for?

An exit pupil between 4 mm and 7 mm delivers the brightest wide-field views under dark skies. Exit pupils above 7 mm waste light because the average dark-adapted eye only dilates to about 7 mm. For high-power planetary work, 1–2 mm is common. Below 0.5 mm, image quality degrades noticeably.

Does TFOV change if I use a Barlow lens?

Yes. A 2× Barlow doubles the effective focal length of your telescope, doubling magnification and halving the TFOV. Enter the new effective focal length (telescope focal length × Barlow factor) in this calculator to account for it.

Eyepiece Field of View Calculator

Name: Eyepiece Field of View Calculator
Availability: InStock
Author: Nham Vu

Enter your telescope focal length and eyepiece specs to instantly compute true field of view, magnification, and sky coverage for up to four eyepieces.

Telescope & Eyepieces

Telescope

Focal Length (mm)

Aperture (mm) (for exit pupil)

Eyepieces

Results appear here after you calculate.

Summary

Enter your telescope focal length and eyepiece specs to instantly compute true field of view, magnification, and sky coverage for up to four eyepieces.

How it works

Enter your telescope focal length in millimeters.
Add one or more eyepieces, each with its focal length (mm) and apparent field of view (degrees).
Magnification is computed as telescope focal length divided by eyepiece focal length.
True field of view equals the eyepiece apparent FOV divided by the magnification.
Exit pupil is derived from your telescope aperture divided by the magnification.
The sky-coverage bar shows how each TFOV compares to the full Moon diameter (0.52°).

Use cases

Plan which eyepiece gives the best view of the full Moon or large nebulae.
Find the highest usable magnification for planetary observing.
Compare a new eyepiece purchase against eyepieces you already own.
Determine whether a wide-field eyepiece covers an entire star cluster.
Check exit pupil suitability for your observing site (urban vs. dark sky).
Quickly verify TFOV without doing mental arithmetic at the eyepiece.

Frequently Asked Questions

Related tools

Telescope Fov Calculator Telescope Magnification Calculator Focal Ratio Calculator

Last updated: 2026-05-29 · Reviewed by Nham Vu