🔠Telescope True Field of View Calculator
Enter your telescope's focal length, your eyepiece's focal length, and the eyepiece's apparent field of view to calculate the true field of view you'll see through the eyepiece.
How the true field is worked out
The eyepiece’s apparent field is divided by the magnification the eyepiece produces in your telescope:
magnification = telescope focal length ÷ eyepiece focal length
true field = apparent field ÷ magnificationWith the opening values — 1000 mm, a 25 mm eyepiece and a 52° apparent field — the magnification is 40× and the true field is 52 ÷ 40 = 1.300°, printed also as 78.0 arcminutes. The full Moon spans about 0.5°, so roughly two and a half Moons would sit side by side in that view. All three inputs must be greater than zero.
Reading the numbers
Apparent field is a property of the eyepiece design, printed on the barrel or in its specification:
- Kellner and older Plössl designs: 50–52°
- Modern wide-field: 68–72°
- Ultra-wide and hyper-wide: 82–100°
A 1.3° field frames the Pleiades tightly and swallows the Orion Nebula whole; Andromeda, at some 3° long, will not fit at any power in a 1000 mm scope. To hold field while gaining power, buy apparent degrees rather than millimetres: a 68° 15 mm eyepiece in that same tube gives 66.7× and still 1.020°, where a 52° 15 mm gives the same power in only 0.780°.
Frequently asked questions
How much sky will I actually see?
Compare the answer with 0.5°, the width of the Moon. A 1.3° field shows about two and a half Moon widths; a 0.25° field cuts the Moon in half.
Does a wider apparent field mean more sky?
Yes, at the same magnification. Swapping a 52° eyepiece for an 82° one of identical focal length leaves the power unchanged and widens the true field by 58%.
What apparent field should I enter if I do not know mine?
52° is a safe guess for a plain eyepiece supplied with a telescope. Look up the model before relying on the figure, as the answer scales directly with it.