Calculate the true field of view (TFOV) for any eyepiece and telescope combination. See how many Moon diameters fit in your view.
Enter your eyepiece's apparent field of view (AFOV) — printed on the barrel, typically 50–52° for a Plössl and 68–100° for wide-field designs — plus the telescope and eyepiece focal lengths in millimeters.
True field of view is computed as AFOV × eyepiece focal length ÷ telescope focal length. A 52° eyepiece at 25mm on a 1200mm scope shows 52 × 25 / 1200 ≈ 1.08° of real sky.
The result is compared against the Moon's roughly 0.5° apparent diameter, so a 1.08° field fits about two full Moons side by side. The AFOV method is an approximation; the eyepiece field-stop method is slightly more exact.
Apparent field of view (AFOV) is the angle the eyepiece presents to your eye, fixed by its design. True field of view is the actual patch of sky you see — AFOV divided by magnification — so the same eyepiece shows less sky on a longer focal length telescope.
The Moon's disk spans roughly 0.5°, so dividing your true field by 0.5 gives the count. A 1.08° field fits about two Moons across, while a planetary setup with a 0.25° field shows only half the lunar disk at once.
Use a longer focal length eyepiece, an eyepiece with a larger apparent field, or a telescope with a shorter focal length. Each change raises the AFOV × eyepiece ÷ telescope ratio that sets the true field.
It is a convenient approximation that ignores eyepiece distortion. Measuring the eyepiece field-stop diameter, or timing how long a star takes to drift across a stationary field, gives a more exact figure when framing is critical.