Telescope Magnification Calculator
Enter your telescope's focal length and the eyepiece you plan to use to see the power, how bright the view will be and how much sky fits in it.
Telescope magnification
Power, exit pupil and true field for a telescope and eyepiece.
Diameter of the main lens or mirror.
Plössl 50 to 52°, wide field 68 to 82°.
Magnification
40x
Low power. Good for wide star fields, large nebulae and finding objects.
- Exit pupil
- 3.25 mm
- True field of view
- 1° 18′
- Focal ratio
- f/7.7
- Useful range
- 19x to 260x
The full Moon is about 0.5° across, so it fits in this view.
Most nights the air limits useful power to about 250x, whatever the telescope.
How this is calculated
- 1
Divide the focal lengths
Magnification is the telescope's focal length divided by the eyepiece's. A 1000 mm telescope with a 25 mm eyepiece gives 40x. A Barlow multiplies the telescope's focal length first, so a 2x Barlow doubles the power.
- 2
Exit pupil
The aperture divided by the magnification gives the width of the beam of light leaving the eyepiece. Around 5 to 7 mm suits a dark-adapted eye; under 0.5 mm the view becomes dim and murky.
- 3
True field of view
The eyepiece's apparent field divided by the magnification is how much real sky you see. It is an approximation that is close enough for planning; the exact figure depends on the eyepiece's field stop.
- 4
Useful range
As a rule the highest useful power is about 2x per millimetre of aperture, and the lowest is the aperture divided by 7, where the exit pupil matches a fully open pupil.
Frequently asked questions
How do I calculate telescope magnification?
Divide the telescope's focal length by the eyepiece's focal length. Both are printed on the equipment in millimetres. For example, 1200 mm divided by 10 mm is 120x.
What is the maximum useful magnification?
About twice the aperture in millimetres, so 260x for a 130 mm telescope. Beyond that you get a larger but blurrier image. In practice the atmosphere usually limits you to 200 to 250x.
What magnification do I need to see Saturn's rings?
The rings are visible from about 25x to 30x, and they look clearly separated from the planet at 75x to 100x. Around 150x shows the Cassini division on a steady night.
Does a Barlow lens reduce image quality?
A good Barlow adds very little. It also increases eye relief with short eyepieces, which makes them more comfortable. A cheap plastic Barlow can soften the image, so it is worth buying a decent one.