UnitConv

Telescope Field of View Calculator

Compute magnification, true field of view, exit pupil and resolution for any telescope + eyepiece, and visualize whether a target fits in the view

Telescope presets

Telescope

Eyepiece

Magnification
48×
True field of view
1.042°
62.5′
Exit pupil
3.13 mm
Focal ratio
f/8
Resolution (Dawes)
0.77″
Rayleigh 0.92″
Useful magnification range
21–300×

The exit pupil is in the ideal range, balancing brightness and detail well for this view.

Field-of-view simulation

See whether the selected object fits inside your true field of view (TFOV), drawn at the same angular scale.

True FOV: 62.5′Moon: 31′

Moon fills about 25% of the field of view.

Sources

What is the Telescope Field of View Calculator?

This calculator combines your telescope (focal length and aperture) and your eyepiece (focal length and apparent field of view) to give the magnification, true field of view (TFOV), exit pupil, focal ratio, resolution and useful magnification range at once. Its standout feature is the field-of-view simulation: the selected target (Moon, Sun, Andromeda Galaxy M31, Orion Nebula M42, Pleiades M45, Hercules Cluster M13, Jupiter or Saturn) is drawn over the true-field circle at the same angular scale. Swap eyepieces and you instantly see whether the object fits or spills over, so you can plan your gear before you observe.

How to use it

1. Pick a telescope preset, or enter the objective focal length (mm) and aperture (mm). 2. Enter the eyepiece focal length (mm) and apparent field of view (°), and a Barlow factor if you use one. 3. Read the magnification, true field of view, exit pupil, focal ratio, resolution and useful magnification range. 4. Choose a target to see its apparent size drawn over the true-field circle and whether it fits.

Formulas

Magnification = telescope focal length ÷ eyepiece focal length (multiply by the Barlow factor if used). True field of view (TFOV) = apparent field of view ÷ magnification. Exit pupil = aperture ÷ magnification (= eyepiece focal length ÷ focal ratio). Focal ratio = telescope focal length ÷ aperture. Resolution (Dawes' limit, arcsec) = 116 ÷ aperture (mm); the Rayleigh limit is slightly larger. Example: a 1200mm, 150mm telescope with a 25mm eyepiece (50° apparent field) gives 48× magnification, a true field of about 1.04° and an exit pupil of about 3.1mm.

How to read the results

Magnification is how much the image is enlarged. The true field of view is how much sky you actually see; lower power gives a wider field, easier for nebulae, clusters and the Moon. The exit pupil is the width of the light beam reaching your eye, ideally 0.5–7mm. Larger means brighter, but above about 7mm (wider than the adult pupil) light is wasted; too small looks dim and diffraction-softened. Resolution shows the finest double star you can split. The maximum useful magnification is roughly twice the aperture in mm; beyond that the image just grows dimmer and blurrier.

FAQ

How is magnification determined? ▾

Magnification equals the telescope's focal length divided by the eyepiece's focal length (multiplied by the Barlow factor if one is used). For example, a 1200mm telescope with a 25mm eyepiece gives 48×. You change magnification simply by swapping eyepieces.

What is the true field of view (TFOV)? ▾

The true field of view is the actual angular patch of sky you see through a given eyepiece. It equals the eyepiece's apparent field of view divided by the magnification. Lower magnification widens the true field, making it easier to frame the whole Moon or a large cluster or nebula.

What is a good exit pupil value? ▾

The exit pupil (aperture ÷ magnification) is practical from about 0.5 to 7mm. At low power it is large and bright, but beyond the adult eye's maximum pupil (~7mm) the extra light is wasted. At high power it is small and dim, with diffraction softening the image. For faint objects, 2–4mm is a common sweet spot.

What is the maximum useful magnification? ▾

As a rule of thumb, the maximum useful magnification is about twice the aperture in millimetres (around 300× for a 150mm scope). You can push higher, but the image only grows dimmer and blurrier, so fine detail is actually harder to see. Atmospheric turbulence (seeing) also caps the usable limit.

What is a Barlow lens? ▾

A Barlow lens sits in front of the eyepiece and lengthens the effective focal length, multiplying magnification by 2×, 3×, and so on. A single Barlow effectively doubles your eyepiece collection's magnification options. This calculator multiplies by the Barlow factor and reflects it in magnification, true field and exit pupil.