Enter the focal length and the eyepiece diameter (in the same units) into the calculator to determine the total magnification of the telescope.

Telescope Magnification Formula

The following formula is used to calculate the total magnification of a telescope.

M = FL / D 
  • Where M is the magnification
  • FL is the focal length
  • D is the eyepiece diameter

To calculate the telescope magnification, divide the focal length by the eyepiece diameter.

Telescope Magnification Definition

What is telescope magnification? A telescope magnification is a measure of the total enlargement of an image that occurs through the lens of a telescope. For example, if a telescope increases the size of the moon by 10 times the normal eyesight size, then the total magnification is 10.

Example Problem

How to calculate telescope magnification?

  1. First, determine the focal length of the lens.

    The focal length is a measure of the distance from the center of the lens to the foci of the lens. For this example, the focal length is found to be 500mm.

  2. Next, determine the diameter of the eye piece.

    This eyepiece is measured and found to have a diameter of 10mm. It’s important that the focal length and diameter are in the same units.

  3. Finally, calculate the telescope magnification.

    Using the formula above, we find the magnification to be 500/10 = 50 times magnification.

Telescope Magnification

What is a good magnification for a telescope? The best magnification for a telescope is as large as possible. The larger the magnification, the large and clearer the image. There is a point where two large magnifications would not be good for looking at something like the moon, but this would happen for personal telescopes.

What magnification do you need to see planets? To see a planet, such as mars, in good detail, you would need a magnification of 100X with an aperture of 5-8″.

telescope magnification calculator
telescope magnification formula