Eyepiece True Field of View Calculator

How it works

True FOV ≈ apparent FOV ÷ magnification; more exactly true FOV = field stop diameter ÷ telescope focal length × 57.3°; drift time (s) = arc-minutes × 4 near the celestial equator

The apparent field printed on an eyepiece (50°, 68°, 82°, 100°) is how wide the view looks to your eye; the true field is the patch of sky it actually shows, which shrinks as magnification rises. Dividing apparent field by magnification is the quick estimate, but eyepiece designs distort the edge of the field, so the manufacturer’s field-stop diameter — the physical ring that clips the view — gives the accurate answer: field stop over telescope focal length in radians. The practical ceiling is the barrel: a 1.25-inch eyepiece cannot have a field stop much over 27 mm, a 2-inch over 46 mm, so the widest true field a telescope can show is fixed by its focal length and barrel size regardless of eyepiece. Wide fields frame large clusters and nebulae and make finding objects easier; narrow fields at high power need tracking or a quick hand.

Worked example

A 25 mm, 68° eyepiece in a 1,200 mm telescope: 48× and a true field of 1.42° (85′) — nearly three Moon-widths, enough to frame the Orion Nebula or the Double Cluster; an undriven star crosses the field in about 5.7 minutes.

Frequently asked questions