Telescope Field of View Calculator
True field of view is the apparent field of the eyepiece divided by the magnification. A 52 degree eyepiece at 48x shows 1.08 degrees of sky, which is about twice the width of the full Moon. For a camera, field of view in degrees is 57.3 multiplied by the sensor dimension in millimetres and divided by the telescope focal length.
Field of view is how much sky you can see at once, and it is the number that decides whether you can find anything. Two formulas cover both ways of looking: through an eyepiece, true field equals apparent field divided by magnification, and on a sensor, field in degrees equals 57.3 times the sensor dimension divided by the telescope focal length. The calculator does both, and reports the result against the width of the full Moon, which is the only ruler in the sky everybody already knows.
Field of view calculator
Switch between the eyepiece view and the camera view. The full Moon is 0.52 degrees across, so the Moon count on the right is the honest way to picture the result.
How do you calculate true field of view?
True field of view is the apparent field of the eyepiece divided by the magnification. Apparent field is a property of the eyepiece design, printed in its specification: a simple Plossl is 50 to 52 degrees, a wide field is 60 to 68, and an ultra wide is 82 or more. Magnification is telescope focal length divided by eyepiece focal length, so the whole calculation runs in two steps.
True field = apparent field ÷ (telescope focal length ÷ eyepiece focal length)
Worked through: a 1200 mm telescope with a 25 mm eyepiece gives 48x. A 52 degree eyepiece at 48x gives 52 divided by 48, or 1.08 degrees. The full Moon is 0.52 degrees across, so the Moon would take up about half the width of that view.
There is a more precise method used by eyepiece manufacturers, based on the physical field stop diameter inside the eyepiece barrel: true field equals 57.3 times the field stop diameter divided by the telescope focal length. It is more accurate because apparent field figures are often rounded generously by marketing departments. Field stop diameters are published for good eyepieces and rarely for cheap ones, which tells you something in itself.
| Eyepiece | Apparent field | Mag in 650 mm | True field | Mag in 1200 mm | True field |
|---|---|---|---|---|---|
| 32 mm | 52° | 20x | 2.56° | 38x | 1.39° |
| 25 mm | 52° | 26x | 2.00° | 48x | 1.08° |
| 20 mm | 68° | 33x | 2.09° | 60x | 1.13° |
| 15 mm | 68° | 43x | 1.57° | 80x | 0.85° |
| 12.5 mm | 60° | 52x | 1.15° | 96x | 0.63° |
| 10 mm | 52° | 65x | 0.80° | 120x | 0.43° |
| 9 mm | 60° | 72x | 0.83° | 133x | 0.45° |
| 6 mm | 68° | 108x | 0.63° | 200x | 0.34° |
| 5 mm | 52° | 130x | 0.40° | 240x | 0.22° |
How much sky is a degree?
Degrees are abstract until you attach them to something. Two references make the whole scale concrete: the full Moon is about half a degree across, and your closed fist held at arm length covers about ten degrees. Everything else follows from those two.
| Object | Apparent size | What it means at the eyepiece |
|---|---|---|
| Jupiter at opposition | 0.8' | Under one arcminute. Field of view is irrelevant, magnification is everything |
| Saturn including rings | 0.7' | Smaller than most people expect. It looks tiny at 50x |
| Mars at opposition | 0.4' | A very small disc even in a large telescope |
| M57, the Ring Nebula | 1.4' | Takes high power well because it is small and bright |
| M13, Hercules Cluster | 20' | Two thirds of the Moon. Comfortable at 100x to 200x |
| The full Moon | 31' | The reference ruler for everything else |
| M42, the Orion Nebula | 85' | Nearly three Moons wide. Wants a one degree field |
| M45, the Pleiades | 110' | Almost two degrees. Most telescopes cannot fit it |
| M31, Andromeda | 190' | Over three degrees. Binocular territory, not telescope territory |
| The Hyades | 330' | Five and a half degrees. Naked eye and binoculars only |
That table settles a question beginners ask constantly, which is why a telescope makes some famous objects look worse than binoculars do. The Pleiades and Andromeda are simply larger than a telescope field. A telescope is the wrong instrument for them, and a reflex finder or a pair of binoculars is the right one. That is not a failing of the telescope, and knowing it in advance prevents a disappointing first night.
How do you calculate camera field of view?
A sensor is a fixed rectangle, so the arithmetic is simpler and the answer is a rectangle rather than a circle:
Field in degrees = 57.3 × sensor dimension in mm ÷ focal length in mm
The 57.3 is the small angle approximation, 180 divided by pi, and it is accurate to well under one percent for any field a telescope produces. Work it once for the sensor width and once for the height. An APS-C sensor, 23.5 by 15.6 mm, behind a 600 mm telescope covers 2.24 by 1.49 degrees. The same sensor behind a 2032 mm telescope covers 0.66 by 0.44 degrees, which is barely larger than the Moon.
The other figure imagers need is image scale, the number of arcseconds each pixel covers, which is 206.265 times the pixel size in microns divided by the focal length in millimetres. A camera with 3.76 micron pixels behind a 600 mm telescope gives 1.29 arcseconds per pixel. Sampling somewhere between one and two arcseconds per pixel suits most sites, since finer sampling than the seeing supports just spreads the same detail over more pixels and lengthens every exposure.
| Sensor | Size | Field at 250 mm | Field at 600 mm | Field at 2032 mm |
|---|---|---|---|---|
| Full frame | 36 x 24 mm | 8.25 x 5.50° | 3.44 x 2.29° | 1.02 x 0.68° |
| APS-C | 23.5 x 15.6 mm | 5.39 x 3.58° | 2.24 x 1.49° | 0.66 x 0.44° |
| Micro Four Thirds | 17.3 x 13 mm | 3.96 x 2.98° | 1.65 x 1.24° | 0.49 x 0.37° |
| IMX533, square | 11.3 x 11.3 mm | 2.59 x 2.59° | 1.08 x 1.08° | 0.32 x 0.32° |
| IMX585 planetary | 7.4 x 5 mm | 1.70 x 1.15° | 0.71 x 0.48° | 0.21 x 0.14° |
| IMX224 planetary | 4.8 x 3.6 mm | 1.10 x 0.83° | 0.46 x 0.34° | 0.14 x 0.10° |
Read the last row against the first. A small planetary sensor behind a long telescope covers less than a fifth of a degree, which is why planetary imagers spend so much effort on getting the target onto the chip at all. A wide field imager with a short refractor and a large sensor has the opposite problem and frames whole nebula complexes in one shot. Both are correct, and they are different hobbies wearing the same coat. The astrophotography starter guide covers which one to begin with, and the NPF exposure calculator works out how long you can expose before the stars trail.
How wide a field can a telescope actually give?
There is a hard ceiling, and it comes from the barrel of the eyepiece rather than from anything optical. The widest true field a telescope can produce is limited by the largest field stop that fits: about 27 mm in a 1.25 inch barrel and about 46 mm in a 2 inch barrel. Run those through the field stop formula and the maximum true field is 57.3 times 27 divided by the telescope focal length, or 57.3 times 46 for a 2 inch focuser.
| Telescope focal length | Widest field, 1.25 inch | Widest field, 2 inch | Practical note |
|---|---|---|---|
| 400 mm | 3.87° | 6.59° | Rich field. Fits the Pleiades with room to spare |
| 650 mm | 2.38° | 4.05° | Fits the Pleiades in a 2 inch eyepiece |
| 900 mm | 1.72° | 2.93° | Andromeda still does not fit |
| 1200 mm | 1.29° | 2.20° | Comfortable for most deep sky targets |
| 1500 mm | 1.03° | 1.76° | One degree is the practical floor for finding things |
| 2032 mm | 0.76° | 1.30° | A 2 inch diagonal is close to mandatory here |
Two things follow. First, a 2 inch focuser and diagonal are not a luxury on a long telescope, they are the difference between being able to find targets and not. Second, if you want genuinely wide fields, the answer is a shorter telescope rather than a longer eyepiece, which is exactly why short refractors exist alongside long ones.
Within a given telescope, a wider apparent field eyepiece is the lever you have. An 82 degree eyepiece at a given magnification shows 58 percent more sky than a 52 degree Plossl at the same power, and a 32 mm Plossl gives the widest field a 1.25 inch barrel can physically deliver. Which one to buy depends on whether your telescope tracks: on an undriven Dobsonian a wide field means fewer nudges, and on a driven mount it is a comfort purchase rather than a necessity.
Related tools and charts
- Magnification calculator, the number this one depends on
- Eyepiece calculator, plan a full set with no gaps or overlaps
- Focal ratio calculator, and what f number means for imaging
- Eyepiece to magnification chart
- Best telescope eyepieces
Frequently asked questions
How do you calculate the true field of view of a telescope?
Divide the apparent field of view of the eyepiece by the magnification. A 52 degree Plossl at 48x shows 52 divided by 48, which is 1.08 degrees of real sky, about twice the width of the full Moon. The telescope contributes the magnification and the eyepiece contributes the apparent field, so both matter and neither is enough on its own.
What is a good field of view for a telescope?
For finding things, aim for at least one degree, which is twice the width of the full Moon and enough to recognise a star pattern. For most deep sky observing, half a degree to one degree is comfortable. For planets, field of view barely matters because the target is under one arcminute across, and a quarter of a degree is plenty even at high magnification.
How large is the Moon in a telescope?
The full Moon is about 0.52 degrees across, or 31 arcminutes, and it varies by roughly 12 percent between perigee and apogee. That makes it a useful ruler: if your true field is one degree, the Moon takes up half the width. At 200x in a 52 degree eyepiece the true field is 0.26 degrees and the Moon no longer fits.
Why does my camera see less sky than my eyepiece?
Because a sensor is a rectangle of fixed size and an eyepiece field is a circle whose size you change by swapping eyepieces. A full frame sensor behind a 600 mm telescope covers about 3.4 by 2.3 degrees, while an APS-C sensor behind the same telescope covers about 2.2 by 1.5 degrees. Shorter focal length is the only way to widen a camera field.
Do wide field eyepieces show more sky?
Yes, at the same magnification. An 82 degree eyepiece at 100x shows 0.82 degrees where a 52 degree Plossl at 100x shows 0.52 degrees, which is 58 percent more sky. On an undriven Dobsonian that translates directly into how often you nudge the tube, which is why wide field eyepieces and Dobsonians are so often paired.
What field of view do I need to see the whole Andromeda galaxy?
About three degrees, which is roughly six full Moons side by side. Andromeda is far larger than most people expect and no telescope at normal magnification fits it in one view. The bright core is what a telescope shows. Binoculars, or a camera lens under 300 mm, are the correct instruments for the whole galaxy.
How we choose: we compare published manufacturer specifications, optical figures we can verify, and reviews from owners who have used the equipment under real skies. We do not test gear in person. Never point any telescope, finder or binocular at the Sun without a certified full-aperture solar filter fitted over the front of the instrument.
Recording your own eyepieces, exit pupils and sessions? The Observing & Astrophotography Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.