field of view
The field of view may refer to the angular size of the actual patch of sky being viewed by an instrument (usually less than 1°).
The field of view is also the apparent size of the field stop as seen through an eyepiece (20° to 90°, depending on the eyepiece). The actual field of view is the angle, measured on the sky, from one edge of the eyepiece field of view to the other; approximately equal to the apparent field of view divided by the magnification. The apparent field of view is the field of view that is written on eyepieces. To get the true field of view, the apparent field must be divided by the magnification of the telescope.