Telescope Magnification

Calculate telescope magnification, true field of view, exit pupil, and resolution from focal length, eyepiece, and apparent field of view.

Telescope Focal Length

Eyepiece Focal Length

Apparent Field of View

Tab to move between fields

Magnification

48×

True Field of View

1.083°

Exit Pupil

3.13 mm

True FOV65 arcmin

Dawes Limit0.77 arcsec

Max Useful Magnification300×

Min Useful Magnification21.4×

Light Gathering Power459.2× eye

Assumed Aperture (f/8)150 mm

How to Use This Calculator

Enter your Telescope Focal Length (mm) — typically printed on the tube or in the manual.
Enter the Eyepiece Focal Length (mm) of the eyepiece you are inserting — shorter eyepieces give higher power.
Enter the Apparent Field of View (°) of the eyepiece — standard Plössl ~52°, wide-angle ~68°, ultra-wide ~82°.
Read Magnification (×) and True Field of View (°) — aim for a magnification where Exit Pupil is 2–5 mm for planetary work.
Compare with Max Useful Magnification (2× aperture in mm) and Min Useful Magnification to stay within the telescope's effective range.

How the result changes with Eyepiece Focal Length

Eyepiece Focal Length	Magnification	True Field of View	Exit Pupil
11	109.1×	0.477°	1.38 mm
36	33.3×	1.56°	4.5 mm
65	18.5×	2.817°	8.13 mm
90	13.3×	3.9°	11.25 mm

Telescope Focal Length: Focal length of the telescope optical tube assembly in millimeters
Eyepiece Focal Length: Focal length of the eyepiece (shorter = higher magnification)
Apparent Field of View: Apparent field of view of the eyepiece (budget ~40°, standard ~52°, wide-angle ~68°, ultra-wide ~82°)

Mag = f_telescope / f_eyepiece; TFOV = AFOV / Mag

Identify Input Parameters
Telescope Focal Length = 1200, Eyepiece Focal Length = 25, Apparent Field of View = 52 = 3 input(s) provided
Calculate Magnification
Magnification = mag
48 = 48
Calculate True Field of View
True Field of View
1.083 = 1.083
Calculate Exit Pupil
Exit Pupil
3.13 = 3.13
Calculate True FOV
True FOV
65 = 65
Calculate Dawes Limit
Dawes Limit
0.77 = 0.77