How much sky an eyepiece actually shows: true field = apparent field ÷ magnification, in degrees and arcminutes, with a Moon comparison.
True field ≈ apparent field ÷ magnification. A 50° eyepiece at 120x shows 0.42° of sky, a bit less than one full Moon (0.52°). Wide-angle eyepieces (68-100° apparent) show proportionally more. The eyepiece field-stop method is slightly more exact, but this classic estimate is within a few percent.
The actual patch of sky visible in the eyepiece. Estimate it by dividing the eyepiece's apparent field by the magnification: a 50° eyepiece at 120x shows about 0.42°, roughly 25 arcminutes.
The angular size of the visible circle inside the eyepiece itself, printed on the barrel: Plössls are typically 50-52°, wide-angles 68-82°, ultra-wides up to 100°. Bigger apparent field means more sky at the same magnification.
The full Moon is about 0.52°, the Pleiades about 2°, the Andromeda galaxy about 3° long. For large objects you need a low-power, wide-field combination; this calculator tells you if the target fits.