How is magnification calculated?

Magnification equals the telescope focal length divided by the eyepiece focal length. A Barlow lens multiplies this result. For example, a 1000 mm scope with a 10 mm eyepiece gives 100x.

What is exit pupil and why does it matter?

Exit pupil is the diameter of the light beam leaving the eyepiece (aperture divided by magnification). A value between 0.5 mm and 7 mm is ideal. Too small causes a dim, hard-to-center image; too large wastes light on the iris of your eye.

What is true field of view?

True field of view is the actual patch of sky visible through the eyepiece, calculated as the apparent field of view divided by the magnification. It tells you how wide a region of sky you will see.

What is limiting magnitude?

Limiting magnitude is the faintest star your telescope can theoretically detect. It is estimated as 2.1 + 5 × log10(aperture in mm). Larger apertures gather more light and reveal fainter objects.

What is the maximum useful magnification?

The practical upper limit is approximately 50x per inch of aperture, or about 2x per millimeter of aperture. Beyond this, atmospheric turbulence and diffraction blur the image more than extra magnification reveals.

What eyepiece apparent field of view should I use?

Standard Plössl eyepieces have about 52°. Wide-angle designs range from 68° to 100°. Check the specifications of your specific eyepiece for the most accurate result.

Telescope Magnification Calculator

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

Telescope & Eyepiece

Telescope Focal Length (mm)

Aperture / Objective Diameter (mm)

Eyepiece Focal Length (mm)

Eyepiece Apparent FOV (degrees) — default 52°

Barlow Multiplier — 1 = no Barlow

Enter telescope and eyepiece values, then click Calculate.

Summary

This tool calculates key optical performance metrics for any telescope and eyepiece combination. Enter your telescope focal length, aperture, and eyepiece focal length to get magnification, exit pupil, true field of view, and limiting magnitude in one click.

How it works

Enter your telescope focal length in millimeters (found on the scope's body or manual).
Enter your telescope aperture (objective diameter) in millimeters.
Enter your eyepiece focal length in millimeters (printed on the eyepiece barrel).
Optionally enter the eyepiece apparent field of view in degrees (default 52°).
Optionally enter a Barlow lens multiplier (e.g. 2 for a 2x Barlow).
Results update instantly — magnification, exit pupil, true FOV, and limiting magnitude appear in the results panel.

Use cases

Compare multiple eyepieces to choose the best magnification for a target.
Find the maximum useful magnification before your image degrades.
Calculate exit pupil to ensure comfortable eye relief in dark conditions.
Determine how much sky fits in the field of view for star clusters or nebulae.
Estimate the faintest stars your scope can resolve from a dark site.
Plan which eyepieces to pack before a night of observing.
Evaluate whether a Barlow lens will exceed your scope's useful magnification limit.
Teach students how telescope optics relate to observational astronomy.

Frequently Asked Questions

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