Telescope Magnification Calculator: Focal Length & Power

Telescope Magnification Calculator

Calculate the magnification level of your telescope based on its focal length and the eyepiece being used.

Telescope Focal Length (mm)

Eyepiece Focal Length (mm)

Total Magnification 40.0x

Understanding Magnification

Magnification in a telescope is not fixed; it is determined by the combination of the telescope's focal length and the eyepiece's focal length. The simple formula is: Magnification = Telescope Focal Length / Eyepiece Focal Length.

Is More Magnification Always Better?

Contrary to popular belief, higher magnification is not always better. As magnification increases, the image becomes dimmer and narrower, and atmospheric turbulence becomes more apparent. The "maximum useful magnification" is generally considered to be about 50x per inch (or 2x per mm) of the telescope's aperture.

Frequently Asked Questions (FAQ)

What is focal length?

Focal length is the distance from the telescope's main lens or mirror to the point where the light comes to a focus.

What does a Barlow lens do?

A Barlow lens effectively increases the focal length of your telescope (usually by 2x or 3x), thereby increasing the magnification of any eyepiece used with it.

How it Works

Magnification is simply the ratio of two focal lengths: M = F_telescope / F_eyepiece. The objective forms an image at the telescope's focal length, and a shorter eyepiece magnifies a smaller portion of that image. Example: a 1000 mm telescope with a 25 mm eyepiece yields 1000 / 25 = 40×, while a 10 mm eyepiece on the same scope yields 100×. As a rule of thumb, the maximum useful magnification is about 2× per millimeter of aperture (50× per inch) — beyond that the image degrades rather than sharpens.