How is telescope magnification calculated?

Magnification equals telescope focal length divided by eyepiece focal length. For example, a 1000mm telescope with a 25mm eyepiece gives 40× magnification.

What's the maximum useful magnification?

The maximum useful magnification is roughly 2× the telescope aperture in millimeters. Beyond this, images become dim and lose quality due to atmospheric limitations.

What is exit pupil and why does it matter?

Exit pupil is the diameter of the light beam exiting the eyepiece. It should match your eye's pupil size (2-7mm) for optimal brightness and comfort.

How do I choose the right magnification?

Use low magnification (25-50×) for wide views and finding objects, medium (50-150×) for most observing, and high (150×+) for small objects when seeing permits.

Telescope Magnification Calculator

Telescope Focal Length

Telescope Focal Length Unit

Eyepiece Focal Length

Eyepiece Focal Length Unit

Telescope Aperture (Optional)

Aperture Unit

Eyepiece Field of View (Optional)

Barlow Lens

Complete Analysis

Calculate telescope magnification and related optical parameters for visual astronomy. This comprehensive tool helps you choose the right eyepieces and understand your telescope's performance characteristics.

How to Use the Telescope Magnification Calculator

Enter your telescope's focal length.
Enter your eyepiece focal length.
Optionally enter telescope aperture for additional calculations.
Add eyepiece field of view and Barlow lens if applicable.
See complete magnification analysis and recommendations.

Magnification Guidelines

Magnification Range	Best For	Typical Eyepiece
25-50×	Wide field views, finding objects, comets	32mm, 40mm
50-100×	Most deep-sky objects, star clusters	15mm, 25mm
100-200×	Galaxies, nebulae, double stars	6mm, 10mm
200-300×	Planetary details, close doubles	4mm, 5mm
300×+	Planetary fine detail (excellent seeing)	3mm + Barlow

Understanding Telescope Optics

Magnification Formula

Magnification = Telescope Focal Length ÷ Eyepiece Focal Length

With a Barlow lens: Magnification = (Telescope FL ÷ Eyepiece FL) × Barlow Factor

Exit Pupil

Exit Pupil = Telescope Aperture ÷ Magnification

Optimal exit pupil size: 2-7mm for most observers (matching eye pupil size)

True Field of View

True FOV = Eyepiece Apparent FOV ÷ Magnification

This shows how much sky you'll actually see through the eyepiece

Practical Limits

Maximum Useful Magnification: ~2× aperture in mm (atmospheric limit)
Minimum Useful Magnification: Aperture in mm ÷ 7 (exit pupil limit)
Sweet Spot: 1-1.5× aperture in mm for most objects

Eyepiece Selection Tips

Start with mid-range: 15-25mm eyepiece for general use
Add wide field: 32-40mm for finding and wide views
High power option: 6-10mm for planets and details
Consider Barlow: 2× Barlow effectively doubles your eyepiece collection
Match your aperture: Larger telescopes benefit from higher magnifications

Illustration of telescope eyepiece and magnification relationship — Calculate precise telescope magnification — optimize eyepiece selection for visual astronomy and observation planning.

FAQ

How is telescope magnification calculated?

Magnification equals telescope focal length divided by eyepiece focal length. For example, a 1000mm telescope with a 25mm eyepiece gives 40× magnification.
What's the maximum useful magnification?

The maximum useful magnification is roughly 2× the telescope aperture in millimeters. Beyond this, images become dim and lose quality due to atmospheric limitations.
What is exit pupil and why does it matter?

Exit pupil is the diameter of the light beam exiting the eyepiece. It should match your eye's pupil size (2-7mm) for optimal brightness and comfort.
How do I choose the right magnification?

Use low magnification (25-50×) for wide views and finding objects, medium (50-150×) for most observing, and high (150×+) for small objects when seeing permits.