How to Calculate Telescope Magnification

One question we often ask is, what is the magnification of your telescope? If you are new to astronomy, you may not know how to calculate this. Don’t panic; it’s a straightforward calculation and an important calculation to understand as each telescope has the Lowest Useful Magnification and the Highest Useful Magnification.

To work out the telescope magnification, you need the Focal Length of the telescope and Focal Length of the eyepiece. You then divide the focal length of the telescope by the focal length of the eyepiece; this gives you the telescope magnification.

So as you can see it’s an easy calculation but there is a little more to it and its best shown with a real working example.

Telescope Magnification Working Example

Celestron nexstar 4se computerized telescope

Focal Length of the telescope 1325mm
Focal Length of Eyepiece (included eyepiece) 25mm

1325MM / 25MM = 53x Magnification

So, now we know that in our example, the Celestron Nexstar 4SE comes with 53x Magnification.

Telescope Highest Useful Magnification Working Example

However, let’s say we want to up our magnification and push the power of our telescope. We need to find the highest useful magnification; this is easily found on the manufactories site.

Let’s keep on using the Nextsar 4se as our example.

Highest useful magnification 241x

At this point it’s worth pointing that you’re Focal Length of the eyepiece gets smaller for more magnification.

For example a 25mm eyepiece on a 1500mm focal-length telescope would yield a power rating of 75x (1500/25 = 60) While a using a 10mm eyepiece on the same telescope would give 150x (1500/10 = 150)

This is something that drips up a lot of new people into astronomy so the smaller the mm with eyepiece the more the magnification.

Let’s get back to our example we know we have a 241x Highest useful magnification.

Right we can take a big jump as we know we have a lot of space to work with let’s go with a 5mm eyepiece and apply our calculation.

Focal Length of the telescope 1325mm
Focal Length of Eyepiece 5mm

1325MM / 5MM = 265x Magnification

Over our 241x highest useful magnification so we need to come up a little lets go with a 5.5mm eyepiece.

Focal Length of the telescope 1325mm
Focal Length of Eyepiece 5.5mm

1325MM / 5.5MM = 240x Magnification

This is on the edge of the highest useful magnification so we know that the smallest eyepiece with the Celestron nexstar 4se (our example) is a 5.5mm eyepiece. You can apply this example with your own telescope.

Again it’s worth pointing out that there is a lowest useful magnification, it’s a similar process but one for another post this post is all about the power.

What Happens If I Go Above the Highest Useful Magnification?

The results will not be great; in short the image thought the eyepiece will become distorted and fuzzy also known as “point spread function”.

If you want to do more reading on “point spread function” Wikipedia has a great resource explaining it all.

So, avoid the disappointment and calculate the correct eyepiece and have much more fun.

Should I Always Go With Higher Magnification?

It’s easy to think that higher magnification will give you a better view as long as you stay within the highest useful magnification. However, this is not always the case, as other factors can affect your views of the night sky.

Seeing Conditions

If there is turbulence in the atmosphere, also known as astronomical seeing can give a very unstable view, even more, if you are using high magnification. If there is lots of turbulence in the atmosphere, you would be better off going with a smaller magnification. What this does is steady’s the view in your eyepiece, and I would only move up the magnification when turbulence in the atmosphere improves.

The object that you are viewing

If you are viewing a star cluster in the eyepiece, for instance, it can look much better at lower magnification. With higher magnification, the view can look cropped in the eyepiece and much less detail leading to disappointment.


I hope this post shows you how to calculate telescope magnification but goes behind that by giving you some food for thought when you come to buy a new eyepiece.

Bigger is not always better, and I have had some of my brightest and sharpest views with low magnification. Experiment with different eyepieces but don’t exceed the highest useful magnification; you will just be wasting your money.

I wish you clear skies and an enjoyable stargazing experience on your next trip out.