Telescope Magnification Explained

Last updated: 10th June 2020

The magnification power of a telescope essentially indicates how big an object appears inside the telescope’s eyepiece compared to the naked eye view. For example, observing Mars with a magnification of 50x means that the red planet will appear 50 times bigger than if you were to look at it with your eyes. 

So why is a telescope magnification important for stargazing? 

telescope magnification explained

Well… every planets, galaxies, nebulas and stars all require different settings to be observed in the best viewing conditions. So mastering your telescope magnification power will enable you to have greater flexibility as an amateur astronomer. At the end of the day, it is one of the very few characteristics about your telescope that you can manually tweak in order to obtain a better image. Let’s find out how!

Practical Limits: Lowest & Highest Magnification

A common mistake made by people who buy a telescope for the first time is to assume that, since the planets and the galaxies are really far away, you would naturally need to magnify them as much as possible in order to obtain a clear view. 

As everything in astronomy, it’s not quite that simple.

There are many factors that can greatly affect the performance of your telescope magnification power, such as the viewing conditions: the light pollution, the atmospheric turbulence, the temperature, the wind, and more. But the most important factors to keep in mind are your telescope lowest and highest useful magnification.

Higher magnification means that you are zooming in on your target, getting closer and thus reducing the field of view, like a horse wearing eye blinds. This also means that the atmospheric turbulence that interferes with the light coming from the object you’re looking at will show more prominently: the planet will appear fainter and fuzzy. For example, if you’re looking at Saturn at 250x when seeing condition is poor, the view will be very unstable and not enjoyable. Reducing your magnification will steady the view and make the image clearer and brighter.

telescope magnification explained fb

Adjusting your magnification according to your telescope and the seeing conditions can greatly impact your planetary observations.

How to Calculate The Magnification Power of Your Telescope

The magnifying power of a telescope is bounded by the amount of light the telescope aperture is collecting. The other two important value in order to accurately calculate the magnification is your telescope’s focal length and the eyepiece’s focal length. Both of those values are indicated on a label placed on both items. The magnification formula is fairly simple: 

The telescope FL divided by the eyepiece FL = magnification power

Example: Your telescope FL is 1000 mm and your eyepiece FL is 20 mm. 1000/20= 50x! The image seen in your eyepiece is magnified 50 times!

Being able to quickly calculate the magnification is ideal because it gives you a more:

  1. Flexibility, because every celestial object has a different brightness (magnitude) and you may quickly switch between different magnification. 
  2. Reactivity, because the weather may greatly affect your viewing and so you can adjust your magnification swiftly and accordingly.

Note: Your telescope focal length is a fixed value that can not be changed, so the only way to increase or decrease the magnification is by changing the eyepiece or barlow lens.

Lowest Useful Magnification: You can calculate your telescope lowest magnification by multiplying your aperture (in inches) by 3 or 4 times. For a 4-inch telescope, the minimum useful magnification is between 12x and 16x. At this power, you’ll have a larger field of view (FOV) and a brighter image which is ideal for large celestial objects such as galaxies and nebulas.

Highest Useful Magnification: Calculating your telescope maximum useful magnification is pretty simple. If you are working in inches, you multiply that value by 50. So for an 8-inch telescope, the highest magnification you can use is x400. More than that will result in an over-magnified image.


Swipe to increase magnification

Telescope Magnification Chart

Increasing your magnification does not mean that you are increasing the amount of light that your telescope is receiving. In fact, high magnification means mean that the light is spread over a larger surface of your retina in order to make the image appear bigger but this causes a loss in brightness. As a rule of thumb, doubling the magnification reduces the brightness of the image seen in your eyepiece by a factor of four.

At the end of the day, every telescope is different and it’s best to try different magnification level on a given target in order to find what works best for your telescope. Personally, I have created a telescope magnification chart that gives me an idea of what magnification worked well for me in the past for a given scope. It looks like this:

Poor Seeing
Good Seeing
Excellent Seeing

Note: The seeing is a measure of how steady the atmosphere above you is. A turbulent atmosphere results in poor seeing.

I was overly obsessed with magnification when I first started using a telescope but what matter more is the field of view, especially for larger deep-sky objects like the Andromeda galaxy.

At the end of the day, my advice would be to not overthink the numbers too much, just practise and play with your equipment. The knowledge and the math behind your telescope will become second nature as time passes. 

Frequently Asked Questions

What magnification do you need to see planets?

When it comes to planetary observations, the state of the atmosphere above you can greatly affect your viewing conditions. A steady air means you can push to 200 / 250x magnification but a turbulent atmosphere might set you back to 150x or less.

What is the best magnification for a telescope?

The best magnification to use for your telescope depends on the brightness and the size object you are trying to observe. At a minimum, its value will be around 4 times the size of your aperture in inches and at a maximum, 50 times your aperture.

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Hey! I’m Tom Urbain, the founder of I have been obsessed with space from a very young age. When I’m not binge-watching space documentaries, movies or TV shows, I can be found in my backyard, carefully collimating my telescope… ready to observe the universe!

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