How to calculate the field-of-view for telescopes?

First and foremost a Field-of-view said simply is the field of view of how much sky you are able to see, which is measured in degrees. For a quick example, if we had eyes in both front and back of our head, our field of view would be 360 degrees because where going to be able to see everything that is going on around us.

First and foremost a Field-of-view said simply is the field of view of how much sky you are able to see, which is measured in degrees. For a quick example, if we had eyes in both front and back of our head, our field of view would be 360 degrees because where going to be able to see everything that is going on around us.

Shortly said, calculating the field-of-view for a telescope is consisted of two main steps – calculating its magnification and then calculating your own personal true field-of-view. But things get a bit scientific and tricky when we talk about the calculation of the field-of-view, especially with a telescope. So keep reading forward to find out exactly what the whole process of calculating a field-of-view for a telescope is.

Calculating the magnification

First and foremost, the first step to begin calculating your telescope’s field of view is understanding how a telescope works. This is a thing pretty much everybody knows, no matter if he is an astronomer enthusiast or not. If you want to see anything through a telescope, you have to look through an eyepiece, sound simple at first probably but when digging deeper things get a bit more complicated. In order for the light to reach your eyes through this said eyepiece, the telescope has to ‘’receive’’ light through the other end.

That mentioned other end is called an objective lens or an aperture, mainly depending on what the type of telescope we are talking about. If you are working with a reflector telescope, which has built-in glass lens at one end and the eyepiece at the other end, then the so called aperture is called an objective lens.

If although on the other hand, you have some other type of telescope different from a reflector, a reflector, you will have a tube with an open end that points directly up to the sky as well as having an eyepiece to the side, not directly on the other end of the telescope. At the closed side tube, there is a mirror that reflects all incoming light off a secondary mirror and all the way up to the eyepiece, getting received by the user of the telescope.

Regardless of the type of telescope you have, all telescopes have what is known to be a ‘’focal strenght’’. This is basically the distance that light travels from the objective lens or aperture, depending on the type of telescope, to the eyepiece.

To begin calculating your magnification, you can typically find the focal strength specified on the sticker placed at the tube of your telescope. If not, then you would have to make a web research on the settings of the focal strength. All eyepieces also possess a focal strength, which is marked either on the barrel of the eyepiece or on the very top. Like the focal strength of a telescope, this is a measurement measured in centimeters also. Unlike a telescope, the focal strength of the eyepiece typically ranges at around 3mm all the way up to 40mm.

So to calculate the magnification of your eyepiece, you should start by dividing your telescope’s focal strength with the focal strength of the eyepiece you have. Obviously, a different eyepiece will give you a different magnification as the calculation changes, depending upon which telescope you are using it with. Similarly, it’s a very good idea to have a small selection of the best eyepieces possible on the market, to give you a range of a higher magnification when you are using your telescope for observing. A higher focal strength of the eyepiece and higher focal strength of the telescope’s tube basically result in better magnification and wider field-of-view.

Calculating field-of-view

This is the second step, where things get a bit complicated – the telescope’s field-of-view calculator. Your eyepiece is known to have an Apparent Field-of-view value, that is typically written down on a sticker on the eyepiece. This is the number of degrees of sky your eyepiece would be able to show its user if you held it directly to your eye, without the use of a telescope. Obviously, this wouldn’t be very effective because there would be no telescope to magnify and focus your view.

The true field of view is the number of certain degrees that your eyepieces shows when you use it, combined with the telescope. To be able to calculate this, you have to divide the apparent field of view by the magnification.

For a quick example, if you have an eyepiece that can vary its focal length from 24mm to 8mm you set the focal strength of 24mm then it will result in a magnification of 27x while having an apparent field of view of 60 degrees. After that dividing 60 degrees by 27 gives the user their true field of view which in our example case is around 2.2 degrees or pretty much the width of two full moons.

Unfortunately for the user, depending on the chosen telescope, not all eyepieces will have the apparent field of view specified or written anywhere on it. The user will probably have to refer to the eyepiece manufacturer or the telescope, original packaging by looking it up further online. If although you aren’t able to find the information required, taking an apparent field of view of 50 degrees as an instance will most of the times suffice.

Other factors to consider when calculating the field of view of a telescope

Two factors that you won’t lose your time taking a look at are the Exit Pupil and The Power-per-inch of the telescope.

The power-per-inch is good to be known and aware of, because it is a method used amongst amateur astronomers that the power-per-inch or PPI for short of one telescope to work properly should not exceed 50 PPI in perfect seeing conditions. In average conditions with a sky with denser clouds, the perfect PPI would have to be around 30 as a practical maximum. PPI can be simply calculated by diving the magnification of the telescope plus the magnification of the eyepiece by the telescope’s clear aperture in inches (taking into consideration that one inch equals around 25 mm).

The Exit Pupil is basically the diameter of light that emerges from the eyepiece. The pupil of a fully adapted eye in darkness can dilate to around 7 millimeters. This leads us to a conclusion that an exit pupil in excess of more than 7 millimeters is passing more light than our eyes can accept. In the meantime, as the exit pupil decreases below the said 7 millimeters, the lack of light becomes the only limiting factor to what the user can see at night through the telescope.

Exit pupils that have less than or around 0.5 millimeters are so small and pass so little light that the eye cannot function properly, making this millimeter range useless. So basically for the proper usage of a telescope you need anything more than 0.5 millimeters at the very minimum. With that said, for a decent viewing quality, the exit pupil would be recommended to be at or above 1.0 millimeters.

Let’s discuss why calculating your field-of-view correctly is a crucial and very important step. Basically, with a larger field of view you can see more of the sky through the specific equipment you are using. With a smaller field of view, you will logically see less sky. As one taken general rule, the lower the magnification is the larger the field of view will be and vice-versa the higher the magnification, the smaller the field of view.


Arguably then, leaving aside those discussed factors, how to calculate your field of view properly really depends on what you are willing to observe. Small Objects, such as galaxies, globular clusters, planetaries and even planets themselves, can be easily observed with a much smaller field of view as we see them in great distance and we actually see them very small – as long as there is a nearby star to focus on around that said object.

Wider objects like star clusters and nebulae often require a much wider field of view because they are typically much larger and are best to be observer under lower magnification so that the user can get a clear picture.

With all that being said, at the end of the day (or in our case in the end of the night) no matter what you are observing and no matter what equipment you choose to use, properly setting up and calculating your field of view is crucial for the best possible stargazing experience.

Related questions

  1. What is field of view in telescopes? –  The field of view is the circle of sky that is visible through the telescope’s eyepiece. Generally speaking, as you change through different eyepieces to get a higher magnification, the field of view gets more and more narrow and you literally see a smaller piece of the sky
  2. How is field of view measured? – The focal length is the distance that Is measured in millimeters, from the point where the light rays converge themselves into the lens. The field of view or FOV for short is used as a means of determining the length that the lens will cover at a specific distance. The FOV can be measured horizontally diagonally or even vertically.
  3. What is your Field of view or FOV in real life? – Individually, every person’s eyes have the capability of having a horizontal Field of view of around 135 degrees and a vertical field of view right over 180 degrees. When the monocular fields of view are stitched together, the binocular when adding an additional sight-improving device, gives us around 114 degrees of horizontal view, which is necessary for depth perception.

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