Telescope Field of View Calculator

Telescope presets

Telescope

Focal length (mm)

Aperture (mm)

Eyepiece

Eyepiece focal length (mm)

Apparent field of view (°)

Barlow (×)

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.

The objective forms an image of the sky at its focal plane, fT behind the lens; the eyepiece, fE behind that plane, magnifies it by M = fT ÷ fE. The exit pupil is the objective's aperture shrunk by M, and the true field is the eyepiece's apparent field divided by M.

Field-of-view simulation

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

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.

Eyepiece numbers, the field-of-view model, and where the simple formula stops being true

Everything below describes exactly what this page computes: the eyepiece figures it starts from, the two ways a true field of view can be worked out, and the points where the answer on screen and the view at the eyepiece part company. The defaults on this page are a 1200 mm f/8 150 mm Newtonian with a 25 mm eyepiece of 50° apparent field: 48×, a true field of 1.04° (62.5'), a 3.1 mm exit pupil, a Dawes limit of 0.77" and a useful range of 21× to 300×.

Apparent field of view by eyepiece type

Apparent field of view (AFOV) is a property of the eyepiece alone: the angular width of the illuminated circle your eye sees. It is the one number in the formula that you cannot derive, so it has to come from the eyepiece. If you do not know it, the design name is a good first estimate.

Eyepiece design	Typical AFOV	What it is used for
Huygens, Ramsden	30 - 40°	Supplied with entry-level scopes; a narrow, keyhole view
Kellner, modified achromat	40 - 45°	Inexpensive upgrade, good on the Moon and planets
Orthoscopic	40 - 45°	High contrast, long the planetary and double-star standard
Plossl	50 - 52°	The general-purpose default; the 50° assumed on this page
Wide field (Erfle and similar)	60 - 70°	Comfortable framing of clusters and large nebulae
Ultra wide	82°	Immersive view; heavy and expensive
Hyper wide	100 - 110°	The widest made; the eye has to rove to see the field edge

These are ranges by design, not a specification for any one eyepiece. Manufacturers round their AFOV figures, and the way an eyepiece distorts angles near the edge changes what the number means, so treat a published AFOV as accurate to a few degrees. When a figure matters, use the field stop instead - see below.

Two ways to get the true field, and why they disagree

This page uses the classic relation: magnification M = telescope focal length / eyepiece focal length (times the Barlow factor), and true field of view TFOV = AFOV / M. It needs only numbers you already know, which is why every eyepiece chart uses it. The more accurate route is the field stop - the physical aperture inside the eyepiece that cuts off the edge of the view. Its diameter is published for most quality eyepieces, and then TFOV in degrees = 57.2958 x field stop diameter (mm) / telescope focal length (mm). No AFOV, no magnification, no rounding. The two disagree by a few percent. A 25 mm Plossl with a 21.2 mm field stop on a 1200 mm telescope gives 57.2958 x 21.2 / 1200 = 1.01°, while AFOV / M gives 50 / 48 = 1.04°. The field-stop answer is the one that matches the sky.

How this page compares with the other two methods

Three ways to answer the same question, in increasing order of trust and effort.

	This page (AFOV / M)	Field stop	Timing a star
What it needs	Eyepiece focal length and AFOV	Published field stop diameter	A stopwatch and a clear night
Typical error	A few percent, from rounded AFOV	Under one percent	Whatever your timing is worth
Good for	Comparing eyepieces before you buy	Planning an exact framing	Confirming what you really have

To time it: let the telescope drive stop, put a star near the celestial equator on the field edge and time its drift across the centre in seconds. TFOV in arcminutes = 0.2507 x seconds x cos(declination). The 0.2507 is the sidereal rate, 15.041 arcseconds per second, divided by 60.

The targets drawn on the view, and how much they vary

The overlay draws each target at a fixed nominal size. Real angular sizes move with orbits, and that matters most for the Moon and the planets.

Target	Size used here	Real range
Moon	31'	29.4' to 33.5' between apogee and perigee
Sun	32'	31.5' to 32.5' over the year
Jupiter	40"	30" to 50" between conjunction and opposition
Saturn (with rings)	42"	The disk alone is 15" to 20"; the rings span about 2.2 times the disk
M31 Andromeda	178' x 60'	The bright core is only about 60'; the full extent needs a dark sky
M42 Orion	85' x 60'	What you see depends on sky darkness more than on the eyepiece

Planet sizes are given in arcseconds here and converted for the drawing; 60 arcseconds is one arcminute. Deep-sky sizes come from photographic extents, so the visible object at the eyepiece is usually smaller than the circle drawn.

Assumptions, and therefore the limits

AFOV is taken as the exact number you type. Published values are rounded and distortion changes their meaning, so the true field can be a few percent off.
A Barlow is treated as its nominal factor. A real 2x Barlow gives more than 2x when the eyepiece sits farther from its lens, and a 2x can behave as 2.5x or more on a long adapter.
The Dawes limit, 116 / aperture in mm, is an empirical result for two equally bright sixth-magnitude stars. The Rayleigh limit, 138 / aperture, is the diffraction figure at 550 nm. Neither describes planetary detail.
Seeing is not modelled. Typical nights blur the view to 2 - 4 arcseconds, so a 200 mm telescope rarely reaches its 0.58 arcsecond Dawes limit whatever the calculator says.
The useful range shown is the usual rule of thumb: 2x per mm of aperture at the top, and a 7 mm exit pupil at the bottom. The upper figure is optimistic on all but the steadiest nights.
A 7 mm exit pupil assumes a young dark-adapted eye. Pupils shrink with age to about 5 mm by sixty, and in an obstructed telescope a large exit pupil lets the shadow of the secondary mirror appear.

Sources

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.