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What true field of view means
True field of view is how much sky you actually see through a telescope and eyepiece together, measured in degrees. It is the number that decides whether the whole Moon fits in the view, whether a star cluster fills the eyepiece or spills out of it, and how long an object takes to drift across the field when the telescope is not tracking. A wide true field makes finding objects easy. A narrow one makes them hard, even when the telescope itself is excellent.
The formula is simple: true field equals the eyepiece’s apparent field divided by the magnification. Apparent field is a property of the eyepiece alone: a basic Plossl eyepiece shows about 52 degrees, while a wide-field design shows 68 or 82 degrees. Magnification comes from the telescope and eyepiece focal lengths. This calculator combines both steps, so you enter three numbers and get the true field, the magnification, and the drift time across the field.
How this differs from camera fields
This is an astronomy calculation, not a photography one. A camera angle-of-view calculator works out how much of a scene a camera lens and sensor record, and a depth-of-field calculator works out which distances look sharp in a photograph. Neither answers the telescope question, because neither knows about eyepiece apparent fields or the way a telescope’s magnification shrinks the sky. The formula here is specific to visual observing: apparent field divided by magnification, nothing else.
How to use the calculator
- Enter the eyepiece apparent field in degrees, printed on the eyepiece or in its specifications. Use 52 if you do not know it.
- Enter the telescope focal length and the eyepiece focal length in millimetres.
- Read the magnification and the true field in degrees.
- Read the drift time: how long an object takes to cross the whole field without tracking.
- Check the Moon note: the full Moon is about 0.5 degrees wide, so a true field above 0.5 fits it whole.
A worked example
Take a 52 degree eyepiece on a 1000 mm telescope with a 10 mm eyepiece. The magnification is 100x, and the true field is 52 divided by 100, which is 0.52 degrees. The full Moon, at about 0.5 degrees, just fits. The drift time is the true field divided by the sky’s turning rate of 0.25 degrees per minute, which gives about 2.1 minutes: that is how long you can watch the Moon drift across the field before you must nudge the telescope.
Now swap in a 25 mm eyepiece on the same telescope. The magnification falls to 40x, and the true field grows to 52 divided by 40, which is 1.3 degrees. The Moon now fits with room to spare, and the drift time stretches to about 5.2 minutes. That is why low-power eyepieces are the finding eyepieces: they show more sky, they keep objects in view longer, and they forgive a telescope that is not perfectly aimed.
Choosing a field for a target
- Finding objects and sweeping the Milky Way: the widest true field you own, ideally above 1.5 degrees.
- Large clusters and nebulae: 1 to 2 degrees, so the object sits in context with its surroundings.
- The Moon whole: at least 0.6 degrees, to leave a margin around the disc.
- Planets and double stars: a narrow field is fine, because the target is tiny and detail matters more than context.
Limits of the formula
True field equals apparent field divided by magnification is an approximation, and it is most accurate for eyepieces with apparent fields below about 60 degrees. Very wide eyepieces bend the geometry slightly, and the simple division can be off by a few percent. Field stops, eyepiece distortion and your own eye’s position also change the field you actually see. Treat the result as a planning number: it tells you which eyepiece to reach for, and whether the Moon will fit, but the view through the eyepiece is the final check.
One more practical note: a wide true field is not free. Wide-field eyepieces cost more, weigh more, and can unbalance a small telescope. A 52 degree eyepiece at low power already gives a generous field on most telescopes, and it is the right starting point for most observers. Buy wider fields when you know which objects you want them for, not before.
Who needs this calculation
Observers planning a night need it to pick the eyepiece for each target before they go outside. Buyers comparing eyepieces need it, because two eyepieces with the same focal length can show very different amounts of sky if their apparent fields differ. Outreach astronomers need it to promise visitors a whole Moon in the eyepiece, and to know how often they must re-centre it. In every case, true field is the number that turns a magnification into a view.
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