Calculating The Magnification Of A Telescope

How much does a telescope magnify objects?

A telescope is a device that uses lenses to increase the size of distant objects.

Telescopes come in various sizes, from large telescopes that focus light into a single beam to smaller ones that focus light onto a camera or eyepiece.

Calculating The Magnification Of A Telescope

The larger the telescope, the greater its magnification power.

Magnification is measured in powers of ten (10x). For example, a 100mm lens can have a magnification of 10x, whereas a 200mm lens has a magnification factor of 20x.

We look at how magnification is calculated in this article in closer detail, and we also discuss some other factors that affect magnification.

Types Of Telescopes

Before we dive into the calculation of magnification, we are going to remind ourselves of the different types of telescopes out there.

A telescope is just a device for collecting light. There are different types of telescopes depending on their design and purpose. Here are some common types of telescopes:


Refractors are made up of lenses and prisms. They are used primarily for observing stars and planets.

The advantage of refractors over reflectors is that refractors do not require a large amount of space to operate.


Reflectors are made up of mirrors and parabolic surfaces. They are designed to collect sunlight to make bright observations.

Cassegrain Refractor

Cassegrain refractors are similar to regular refractors but have additional optics that allow them to focus images onto the primary mirror.

These telescopes are often used for deep sky observation.

Ritchey-Chretien Reflector

Ritchey-Chrétien reflectors are also known as Schmidt-Cassegrain telescopes. They consist of two separate mirrors that are placed side by side.

One mirror collects the incoming light while the other focuses the light into the eyepiece. Ritchey-Chrétiéns are usually used for planetary observing.

Dobsonian Refractor

The Dobsonian telescope is one of the most popular types of telescopes available today. This type of telescope consists of an equatorial mount that holds the telescope itself.

This allows you to observe any object that is located in the same direction as your telescope.

How Do We Calculate Magnification?

Magnification is simply the ratio between the focal length of the telescope and the distance from the objective lens to the image plane.

In other words, it’s the product of these two numbers.

It is important to note that the focal length of the objective lens is always longer than the distance from the objective to the image plane.

For instance, let’s say we want to calculate the magnification of our 50mm f/1.8 prime lens.

If we know the focal length of the lens, then we can find the distance between the objective lens and the image plane.

Now, if we multiply the focal length of the 50mm lens by the distance between the objective and the image plane, we get a value of about 1,000 mm.

Therefore, the magnification of the 50mm lens is equal to 1,000 divided by 50 which equals 20x.

The Magnification Factor

The Magnification Factor

Magnification is measured in powers-of-ten (10x) as shown above. A telescope with a magnification factor of 10x will magnify an object by a factor of 1/10th.

If you were looking through a telescope with a magnification factor equal to 10x, then the image on your retina would be ten times bigger than what you see without a telescope.

Magnification Factors

There are two types of magnification: real and apparent. Real magnification refers to the actual size of the object being viewed.

Apparent magnification refers to the size of the image seen through the telescope.

Real magnification is, of course, always less than apparent magnification.

Magnification depends on three things: the focal length of the telescope, the distance between the observer and the object, and the aperture of the telescope.

Focal Length

Focal length is the distance from the front of the objective lens to the back of the eyepiece. It determines how far away objects can be before they start appearing blurry.

In general, the longer the focal length, the more magnification power the telescope has.

Telescope Aperture

The diameter of the main mirror inside the telescope determines its overall size.

Smaller mirrors create higher magnifications. Larger mirrors produce lower magnifications.

Distance Between Observer and Object

If you stand too close to an object, you won’t be able to see it clearly because the rays of light coming from the object will not reach your eyes.

You need to be farther away from the object so that all the light rays coming from the object hit your eyeballs.

Magnification Calculation Simplified

The magnification of an astronomical telescope changes with the type of eyepiece that is being used.

It is worked out by dividing the focal length of the telescope by the focal length of the eyepiece.

The focal length of the telescope is often marked on the optical tube and both focal lengths are measured in millimeters. The calculation would look something like this:


This means that a telescope that has a 1000mm focal length and a 10mm ocular attached is showing objects at 100x magnification.

Magnifying Glasses

If you did not want to use a telescope, a pair of glasses called “magnifiers” is used to increase the size of an object.

They consist of a convex lens placed over each eye. When you wear these glasses, the images formed by the left and right lenses are combined on your retinas.

As a result, you see a larger version of the object.

The magnification produced by the magnifying glass is limited by the amount of curvature in the lenses.

For example, if you have a pair of 20mm magnifying glasses, then you will only be able to magnify objects about 20 times their normal size.

On the other hand, if you have a 50mm magnifying glass, then you can magnify objects 100 times their normal size.

The magnification produced by a pair of magnifying glasses is given by this formula:

M -2 * log(1 + f / D)

where M is the magnification, f is the focal length of the magnifying glass, and D is the distance between the observer’s eye and the object.

For example, suppose you want to magnify an object 10 times. The equation would look like this:

10 2 * log (1 + 1/50)

This gives us a value for M of about 11.3. If we substitute our values into the equation, we find that the focal length of the 50mm magnifier is about 0.6 meters.

This means that the object must be at least 60 centimeters away from the observer.

To calculate the distance between the observer’s eye and the object using the equation above, simply divide the focal length of the lens by the magnification.

So, the distance between the eye and the object for the 50mm magnifying glass is about 30 cm.

Now let’s say you wanted to magnify an object 100 times. The equation looks like this:

100 2 * log (1+1/0.006)

This gives us a value for the magnification of about 101.5. Substituting our values into the equation yields a focal length of about 0.06 meters.

This means that when you put the 50mm magnifying glasses on, the object should be 6 centimeters away from your eye.

Final Thoughts

We hope after reading this article you now understand how telescopes work and what they do.

We also hope that you learned some basic terminology associated with astronomical telescopes and most importantly you understand what magnification means and how it is calculated.

Although it seems a little complicated, if you remember the basic formula that magnification is calculated using telescope focal length divided by ocular focal length, you should be good to go!

Gordon Watts