What this calculator does
The field of view through a telescope, the width of sky actually visible in the eyepiece, depends on three things: the telescope's focal length, the eyepiece's focal length, and a spec printed on the eyepiece itself called its apparent field of view. This calculator combines all three into the true field of view, the number that tells you how much sky you will actually see.
A common surprise for anyone calculating field of view for the first time is how much it shrinks as magnification rises. A wide, sweeping view of a star cluster and a tight, zoomed-in view of a planet are not achieved by changing the telescope, but by swapping the eyepiece, which changes the magnification and, in turn, the field of view.
The formula
First find the magnification: telescope focal length divided by eyepiece focal length. Then divide the eyepiece's apparent field of view (a fixed spec of that eyepiece design, typically 40 to 100 degrees, printed by the manufacturer) by that magnification. The result is the true field of view, the actual angular width of sky visible, usually a small fraction of a degree to a few degrees.
| Term | Meaning |
|---|---|
| Telescope focal length | The distance, usually in millimetres, light travels inside the telescope from the main lens or mirror to the point where it comes into focus. Fixed for a given telescope. |
| Eyepiece focal length | A shorter focal length, printed on the eyepiece itself, that determines magnification when paired with a given telescope. |
| Apparent field of view (AFOV) | A fixed spec of the eyepiece design, the angular width the eyepiece itself appears to show, independent of which telescope it is used with. |
| True field of view (TFOV) | The actual width of sky visible when that eyepiece is used in that telescope: apparent field of view ÷ magnification. |
The inputs explained
| Field | What to enter |
|---|---|
| Telescope focal length (mm) | The telescope's focal length, found in its specifications, usually printed in millimetres. |
| Eyepiece focal length (mm) | The eyepiece's focal length, printed on the eyepiece barrel, also in millimetres. |
| Eyepiece apparent field of view (°) | The eyepiece's apparent field of view in degrees, a spec published by the eyepiece manufacturer that varies by eyepiece design, commonly somewhere between 40° and 100°. |
When to use it
Choosing an eyepiece for a target
A large, spread-out object like a star cluster or the Andromeda galaxy needs a wide true field of view to fit in the eyepiece at all, while a small target like a planet benefits from higher magnification and a narrower field.
Working out if a specific eyepiece will show the whole Moon
The full Moon spans about half a degree of sky, so calculating field of view for a candidate eyepiece and telescope combination shows whether the whole disc will fit in view or run off the edge.
Comparing eyepieces before buying
Two eyepieces with the same focal length but different apparent fields of view (say 52° against 82°) will show noticeably different true fields of view in the same telescope, which calculating field of view for both makes concrete before spending money.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How true field of view changes with eyepiece focal length
A fixed telescope, paired with a range of eyepiece focal lengths.
| Eyepiece focal length | Magnification | True field of view | True field of view (arcminutes) |
|---|---|---|---|
| 9 mm | 111.1× | 0.468° | 28.08' |
| 15 mm | 66.7× | 0.780° | 46.80' |
| 25 mm | 40.0× | 1.300° | 78.00' |
| 32 mm | 31.3× | 1.664° | 99.84' |
| 40 mm | 25.0× | 2.080° | 124.80' |
How true field of view changes with telescope focal length
A fixed eyepiece, paired with a range of telescope focal lengths.
| Telescope focal length | Magnification | True field of view | True field of view (arcminutes) |
|---|---|---|---|
| 500 mm | 20.0× | 2.600° | 156.00' |
| 700 mm | 28.0× | 1.857° | 111.43' |
| 1000 mm | 40.0× | 1.300° | 78.00' |
| 1250 mm | 50.0× | 1.040° | 62.40' |
| 2000 mm | 80.0× | 0.650° | 39.00' |
Questions
What is the field of view formula?
True field of view = eyepiece apparent field of view ÷ magnification, where magnification = telescope focal length ÷ eyepiece focal length. Both focal lengths are usually printed in millimetres and the apparent field of view in degrees.
Where do I find my eyepiece's apparent field of view?
It is a spec of the eyepiece design itself, published by the manufacturer, not something you measure. Simple Plössl eyepieces are often around 50°, while wide-field designs can reach 70°, 82° or more. Check the eyepiece's packaging or listing if it is not printed on the eyepiece itself.
Why does calculating field of view matter for choosing an eyepiece?
It tells you, before you buy or before you point the telescope, whether a target will fit in the view at all. A wide object viewed at high magnification with a narrow field of view may not fit in the eyepiece, even though the object itself is bright and otherwise easy to find.
Is true field of view exactly accurate for every eyepiece?
This is the standard approximation used throughout amateur astronomy and is accurate for most eyepiece designs. A small number of wide-angle eyepieces have enough optical distortion that the true field is slightly different from this simple division, though the difference is usually minor for planning purposes.
For the angular width of a Full Moon or planet you are trying to fit in that field of view, see the geometry and physics calculators elsewhere on the site for angle-based sizing problems.