What is the difference between apparent and true field of view?

Apparent field of view (AFOV) is the angular width of the view circle as seen through the eyepiece, a fixed property of the eyepiece design. True field of view (TFOV) is the actual patch of sky you see, which depends on the magnification your telescope gives that eyepiece.

How is magnification calculated?

Magnification equals the telescope focal length divided by the eyepiece focal length. A 1000mm telescope with a 10mm eyepiece gives 100x. Shorter eyepieces magnify more and therefore show a smaller patch of sky.

Why is true field of view useful?

It tells you whether an object will fit in the view. The full Moon is about 0.5 degrees or 30 arcminutes wide, so an eyepiece giving a 1 degree true field frames it comfortably with room to spare. Large objects like the Pleiades need an even wider field.

Is the simple AFOV-over-magnification formula exact?

It is a very good approximation used universally for visual planning. A more precise method uses the eyepiece field stop diameter divided by telescope focal length, but field-stop figures are rarely published, so the AFOV method is the practical standard.

Does a focal reducer or Barlow change the result?

Yes. A Barlow multiplies the effective telescope focal length, raising magnification and shrinking true field. A focal reducer does the opposite. Enter the effective focal length after the Barlow or reducer to get the real field of view.

Eyepiece True Field of View Calculator

The true field of view tells you how much sky you actually see through a given eyepiece on a given telescope — the key number for deciding whether a star cluster or the Moon will fit in the view. This calculator works out magnification and then converts the eyepiece’s apparent field into a true field in degrees and arcminutes.

How it works

Two simple relationships drive the result:

magnification = telescope_focal_length / eyepiece_focal_length
true_FOV      = apparent_FOV / magnification

The apparent field of view is a fixed property of the eyepiece. Dividing it by the magnification your telescope produces gives the true field — the real angular size of the sky circle. Multiplying degrees by 60 converts to arcminutes, which is handy because many deep-sky objects are catalogued in arcminutes.

Example and tips

A 1200mm telescope with a 25mm, 68-degree eyepiece gives 48x and a true field of about 1.42 degrees (85 arcminutes) — wide enough to frame the whole Pleiades cluster. Swap to a 6mm eyepiece for 200x and the field shrinks to roughly 0.34 degrees, good for tight planetary views. Keep a range of eyepieces: low power for sweeping wide fields, high power for splitting double stars and studying planets.