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Field of View in Astronomy Explained

The term field of view confuses a lot of people new to the hobby of astronomy. Here’s what to know about the field of view in optical instruments, including apparent vs true and degrees vs arcminutes, to help you demystify this term and get better satisfaction out of using your optical instruments.

field of view in astronomy explained

Define field of view

Field of view is the extent or the range of your visual area.” ~ Optics Den

In astronomy, the field of view “is the angular width of the patch of sky you can see through your optical instrument” (Mosley, 2003). In binoculars the units are usually expressed as degrees and in telescopes, arcminutes.

Field of view in optical instruments for stargazing

As a rule of thumb, the field of view of optical instruments are as follows:

  • Naked eye — 100º
  • Pair of binoculars — 5º–8º
  • Spotting scope — 2°–1.4°
  • Low power telescope — 0.5º or 30 arcminutes (‘)
  • High power telescope — 0.166º or 10′

Apparent field of view vs True field of view

When it comes to field of view, there’s ‘apparent’ and ‘true’. You might be wondering how these differ. Here are the definitions of true vs apparent field of view:

What is true field of view? The true field of view (TFOV) is what you see of the sky when looking through an optical instrument, either a pair of binoculars, a telescope, or the naked eye.

What is the apparent field of view? The apparent field of view (AFOV) is a fixed property of a telescope eyepiece expressed in degrees. It is the angular diameter of the circular view that’s determined by the eyepiece’s field stop diameter. Compare this to the true field of view (TFOV), which depends on both the telescope’s magnification and the eyepiece AFOV.

best eyepiece, field of view
Diagram showing the field of view. Source: Randy Culp

field of view in telescope

What is the field of view in a telescope? The field of view in a telescope is the circle of what you see of space when looking through this optical instrument. It’s determined by the telescope’s power and the apparent field of view of the eyepiece. It is typically expressed as arcminutes.

How to calculate TFoV of a telescope – Method 1:

  • True FoV = Apparent FoV ÷ magnification;
    • Where magnification is the telescope focal length ÷ eyepiece focal length. The steps:
      • TFL÷EFL… where the telescope focal length, say 900mm, is divided by the eyepiece focal length, say 20 mm, to give magnification, 45X.
      • AFoV÷magnification… where the AFoV, say 100°, is divided by the magnification, 45X.
      • TFoV is 100/45, giving result of ~2.22° (or 133.3 arcminutes).

The size of the field stop determines the AFoV of the eyepiece. And so, there’s another formula to calculate TFoV for a telescope, based on the field stop dimensions…

Field of View (FoV) Calculation for Telescope – Method 2

TFOV = eyepiece field stop diameter ÷ telescope’s focal length x 57.3

Few brands list this field stop dimension, apart from TeleVue.

Why care about AFoV of eyepieces?

Why does anyone care about the apparent field of view? Isn’t the true field what really matters? Let’s compare eyepieces with the same TFoV and then with the same AFoV to understand how AFoV matters.

Here’s why AFoV matters: A wider AFoV will give you more sky in your view at a particular magnification.

same TFoV

Imagine you have a normal drinking straw, 5 mm in diameter and about 200 mm in length and you’re looking through it. The AFoV equals the TFoV, i.e., 1.4325º (5 mm x 57.3 ÷ 200 mm — see Method 2), noting that the magnification is 1X. You’ll also get this TFoV of 1.4325º with a 50º (AFoV) Plossl at 35X (magnification), a 70º superwide at 49X, and an 82º ultrawide at 57X (Method 1 — AFoV÷magnification).

All of the above have the same TFoV, but different AFoVs and with each you get enormously different views. Compare what you’d see through a drinking straw (at 1x magnification) to what you’d see with 57X magnification.

Let’s look at it another way…

same AFoV different magnification

Imagine you have two eyepieces with the same AFOV, say 100° but with different focal lengths, say 13 mm and 21 mm (as in TeleVue Ethos eyepieces) and thus different magnification.

Tele Vue 21mm Ethos 2" Eyepiece


TeleVue 21mm Ethos

available at Amazon

Tele Vue 13mm Ethos 2" / 1.25" Eyepiece with 100 Degree Field of View.
TeleVue 13mm Ethos

available at Amazon

The 13 mm gives a view that’s about double the magnification of the 21 mm — The object looks about twice as big through the 13 as opposed to the 21 mm. But because both have a 100° AFoV, the 13 mm will have a smaller TFoV because you are only fitting in half of the image you get with the 21 mm.

Where AFoV separates the eyepieces…

Different AFoV same magnification

If you have two eyepieces with the same magnification but different AFoV, you’d choose the eyepiece with the wider apparent field of view if you want to see more of the sky in your view.

For example, consider (1) an 8 mm Plossl with 50º (AFoV) and a field stop of 6.5 mm vs (2) an 8 mm Ethos with 100º (AFOV) and a field stop of 13.9, both in a telescope with focal length of 900 mm. Here are the calcs :

Compare AFoV of eyepieces

TFoV 1

8 mm Plossl 50º AFoV

eyepiece field stop diameter (6.5) ÷ telescope’s focal length (900) x 57.3

= 0.41º

TFoV 2

8 mm Ethos 100º AFoV

eyepiece field stop diameter (13.9) ÷ telescope’s focal length (900) x 57.3

= 0.88º

You’ll get a much wider view of the sky with the latter unit, about twice as much in fact.

Field of view binoculars

In binoculars, the field of view is expressed as degrees or a linear measure of width (e.g., feet @ 1000 yds). To convert angle to linear feet FOV, multiply it by 52.5 (based on at 1000 yards, one degree is 52.5 feet).

With binoculars you can enjoy a wider field of view, say 5–8º, compared to that of a telescope (< 1º). This FoV corresponds to four fingers at arms length held up to the sky (see my article on hand measures).

BINOCULAR field of view compared for lower vs higher magnification

Basically, as you go up in power the field of view narrows. This is something to weigh up when choosing astronomy binoculars.

Bottom line

I hope the above graphics and content has removed the confusion and shed light on the meaning of apparent field of view property for eyepieces, and then the true field of view in terms of the naked eye, binoculars, and telescopes.

In a nutshell, the TFoV is the circle of sky you see through your optical device, whereas the AFoV is a fixed property of your eyepiece. The thing to note here is that an eyepiece with a wider AFoV gives you a broader view of the sky (wider TFoV) at a particular magnification.

It’s important to understand these features as it helps in choosing the right eyepiece and getting the most satisfaction out of using your telescope to observe planets or other objects or with using binoculars for astronomy.

References

  1. Mosley, John. Starry Night Companion. Space Holding Corp. Canada.