Telescopes
Understanding Magnification and Field of View
Four numbers govern every view through every eyepiece. Once they are intuitive, choosing eyepieces stops being guesswork.
Magnification is the most-quoted and least-useful number in amateur astronomy. It is useful only in relation to aperture, exit pupil and true field, and those relationships are simple enough to hold in your head.
Magnification
Magnification equals telescope focal length divided by eyepiece focal length. A 1200 mm telescope with a 10 mm eyepiece gives 120×. Add a 2× Barlow and it becomes 240×.
The useful ceiling is roughly 2× the aperture in millimetres, and the useful floor is roughly 0.5× — below that the exit pupil exceeds the pupil of your eye and light is wasted. For a 150 mm telescope that means a practical range of about 75× to 300×, and in practice the atmosphere usually stops you well below the top.
Magnification above the useful ceiling is not "empty magnification" in the sense of showing nothing — it shows a larger image with less contrast, less brightness and no additional detail. On planets, dropping from 300× to 200× often reveals more.
Exit pupil
Exit pupil is the diameter of the beam of light leaving the eyepiece: aperture divided by magnification, or equivalently eyepiece focal length divided by focal ratio. It is the number that actually determines image brightness at the eye.
- Above about 7 mm the beam is wider than a dark-adapted pupil — light is wasted and the view looks no brighter.
- Around 2 mm is the sweet spot for faint deep-sky objects: maximum contrast against the sky background.
- Around 0.7 mm suits planets and the Moon, where the object is bright enough to spare the light.
- Below about 0.5 mm the image dims noticeably and eye floaters become visible, since the beam is smaller than the eye's own imperfections.
| Use | Exit pupil | Typical magnification on 150 mm f/8 |
|---|---|---|
| Maximum brightness, wide field | 6 – 7 mm | 21× – 25× |
| Faint deep sky, best contrast | 2 – 3 mm | 50× – 75× |
| General deep sky | 1 – 2 mm | 75× – 150× |
| Planets and Moon | 0.5 – 1 mm | 150× – 300× |
True field of view
Apparent field is a property of the eyepiece — 50° for a Plössl, 68° to 100° for a wide-field design. True field is what you actually see on the sky: apparent field divided by magnification.
This is where many beginners get caught. An 82° eyepiece sounds dramatically wider than a 52° one, and at equal focal length it is. But a 25 mm Plössl at 52° on a 1200 mm telescope gives a 1.08° true field, while a 10 mm 82° eyepiece at the same 120× gives 0.68° — narrower, despite the far wider apparent field. Magnification eats field.
There is also a hard cap: a 1.25″ barrel limits true field to roughly 1.2° no matter how wide the apparent field, because the field stop cannot exceed the barrel. Truly wide fields need a 2″ eyepiece and a 2″ focuser.
Work out true field before buying a wide-field eyepiece. On a long-focal-length SCT, a 2″ eyepiece and a focal reducer together are what actually deliver a wide view — the eyepiece alone will not.
Resolution and what limits it
Dawes limit — 116 divided by aperture in millimetres, in arcseconds — is the conventional figure for the finest detail an aperture can resolve. A 150 mm telescope gives 0.77″; a 200 mm gives 0.58″.
But aperture is rarely the binding constraint. Atmospheric seeing typically limits resolution to 1–3 arcseconds on an average night, whatever the telescope. Which is why a C11 shows no more planetary detail than a good 6-inch on a mediocre night from a suburb.
Building a working eyepiece set
One low-power wide field — the longest focal length your barrel supports. This is your finder and your large-object eyepiece.
One mid power around 2 mm exit pupil — the workhorse for faint galaxies and nebulae.
One high power around 0.7 mm exit pupil — planets, the Moon and close doubles.
Optionally a 2× Barlow, which turns the mid and low powers into a second set and keeps eye relief comfortable.
Three or four eyepieces cover essentially everything. Resist the twelve-eyepiece case; most of it will not get used.
All figures here are conventional approximations used across the hobby. Real performance depends on optical quality, collimation, seeing and sky brightness.
This article is educational reference material. Figures are typical approximations, and no observing result is guaranteed — your equipment, sky and conditions will differ.