What is magnification in a telescope?

Magnification (power) is how many times larger an object appears compared to the naked eye. It equals the telescope focal length divided by the eyepiece focal length. Higher magnification shows finer detail but narrows the field of view.

What is apparent field of view (AFOV)?

AFOV is the angular size of the image circle as seen through the eyepiece itself, independent of the telescope. Common values are 50° for standard Plössl eyepieces and 82–100° for wide-field designs.

What is true field of view (TFOV)?

True FOV is the actual patch of sky visible through the telescope-eyepiece combination. It equals the eyepiece AFOV divided by the magnification. A TFOV of 1° fits about two full-Moon widths side by side.

What is exit pupil and why does it matter?

Exit pupil is the diameter of the beam of light that exits the eyepiece and enters your eye. It equals aperture divided by magnification. Values between 5–7 mm work best under dark skies; 1–2 mm suits high-power planetary viewing. Values above 7 mm waste light when your eye cannot fully dilate.

What magnification is too high?

A practical rule is 50× per inch of aperture (about 2× per mm). Beyond this, images become dim and blurry due to atmospheric turbulence and diffraction limits.

Telescope FOV Calculator

Name: Telescope FOV Calculator
Availability: InStock
Author: Nham Vu

Enter your telescope and eyepiece specs to instantly get magnification, true field of view, and exit pupil.

Telescope & Eyepiece

Telescope

Focal Length (mm)

Aperture (mm)

Eyepiece

Focal Length (mm)

Apparent FOV (degrees)

Results

Magnification

40x

True FOV

1.30°

Exit Pupil

2.50 mm

True FOV Breakdown

Degrees

1°

Arcminutes

18′

Arcseconds

0″

Telescope Focal Ratio (f/) f/10

True FOV vs. full Moon (0.52°) 2.5x Moon width

0° Full Moon (0.52°) 2.5°

Exit Pupil Quality Good

High power (0.5 mm) Ideal (4–5 mm) Dark sky (7 mm)

Summary

Enter your telescope and eyepiece specs to instantly get magnification, true field of view, and exit pupil.

How it works

Enter your telescope focal length and aperture in millimeters.
Enter the eyepiece focal length and its apparent field of view (AFOV).
Magnification is computed as telescope focal length divided by eyepiece focal length.
True field of view is the eyepiece AFOV divided by magnification, shown in degrees, arcminutes, and arcseconds.
Exit pupil is computed as aperture divided by magnification to gauge image brightness.

Use cases

Choose the right eyepiece for observing the Moon, planets, or deep-sky objects.
Compare multiple eyepiece and telescope combinations before buying.
Determine whether your exit pupil is suitable for your observing conditions.
Plan a star-party by pre-calculating FOV for specific targets.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu