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

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## 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

## 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?

**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.

**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.

**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″.