Magnification is often the first number a new telescope owner looks for — and the most misunderstood. More power is not always better, and the relationship between aperture, focal length, eyepiece, and what you actually see is more nuanced than any single number can capture. This guide explains the complete optics chain so you can choose the right eyepiece for any target, any night.
How Magnification Works
A telescope magnifies because it has a long focal length and an eyepiece has a short one. Light entering the objective (lens or mirror) converges at the focal plane; the eyepiece acts like a magnifying glass to spread that focused image across your retina. The magnification ratio is simply the telescope focal length divided by the eyepiece focal length: a 1000mm scope with a 10mm eyepiece gives 100×.
A Barlow lens sits in the light path before the eyepiece and diverges the converging beam, effectively multiplying the focal length of the telescope. A 2× Barlow doubles the effective focal length and therefore doubles the magnification of any eyepiece. This means two eyepieces and a 2× Barlow give you four magnification options — a cost-effective way to build a versatile eyepiece kit.
The Exit Pupil: Your Most Important Number
Exit pupil is aperture divided by magnification, and it determines how bright your image appears. At night, the dark-adapted human eye dilates to roughly 5–7mm (declining with age — a 20-year-old may reach 7mm; a 60-year-old may manage only 5mm). An exit pupil equal to your dark-adapted pupil diameter gives maximum background sky brightness, ideal for large diffuse objects like nebulae and galaxies.
For lunar and planetary work, you want a smaller exit pupil (1–3mm) to reduce glare and improve contrast. At exit pupils below 1mm, the image dims noticeably and atmospheric turbulence becomes more visible, which is why there is a practical upper limit to useful magnification regardless of what the mathematics allows. The rule of thumb — maximum useful magnification equals twice the aperture in millimeters — reflects this exit-pupil floor of about 0.5mm.
True Field of View and Why It Matters
True field of view is the width of sky visible through the eyepiece. Wide fields (1° or more) let you see entire open clusters, the Pleiades, or the Orion Nebula in full context. Narrow fields (0.3° or less) isolate a single planet, a tight double star, or the detail within a globular cluster's core.
Each eyepiece has an apparent field of view (AFOV) — the angle subtended by the eyepiece's field stop as seen by your eye. Standard Plössl designs give about 50–52°. Wide-angle Erfle types offer 60–68°. Ultra-wide-angle modern designs reach 82° or even 100°, giving a dramatic immersive view. The true field of view equals the apparent FOV divided by the magnification, so a 68° AFOV at 100× gives a 0.68° true field — just wide enough to frame the full disk of the Moon with a little room to spare.
Resolving Power, Aperture, and What You Can Actually See
Resolving power — the ability to separate fine detail — depends entirely on aperture. The Dawes limit (116 / aperture in mm, in arcseconds) gives the theoretical minimum separation for a double star. In practice, atmospheric turbulence (called 'seeing') often limits resolution to 0.5–2 arcseconds regardless of aperture. On nights of poor seeing, even a large telescope performs no better than a smaller one, because the turbulence blurs the image before magnification can reveal the detail.
Limiting magnitude, by contrast, improves steadily with aperture because more light means fainter stars become detectable above the noise of your eye. A 60mm refractor reaches about magnitude 11.1 under perfect skies; a 300mm reflector extends that to magnitude 14.8. Each doubling of aperture adds approximately 1.5 magnitudes — about 4× more stars visible.
Choosing Eyepieces for Different Targets
Moon and planets: Use medium-to-high magnification, typically 100–250× depending on aperture. Exit pupil of 1–3mm. Maximize contrast by using a narrow field; detail comes from resolution, not field size.
Double stars: High magnification, up to or near the maximum useful limit. Pairs closer than 1 arcsecond require excellent seeing and at least 100mm aperture. Wider pairs (5–30 arcseconds) are rewarding targets for modest apertures at 100–150×.
Globular clusters: Medium magnification (80–200×) to resolve the outer stars into pinpoints while keeping the bright core visible. Too low a magnification leaves them unresolved; too high loses the sweeping radial structure.
Open clusters: Low magnification (20–60×) for wide field. Many of the finest open clusters — the Pleiades, the Hyades, the Beehive — are actually too large for high magnification and look best at 20–40× in a 1–2° field.
Galaxies and nebulae: Low-to-medium magnification, 20–100×, to keep extended objects bright. Surface brightness falls as magnification increases (spreading the same light over more of the retina), so pushing power on faint galaxies makes them harder, not easier, to see.