Telescope Eyepiece Magnification Chart

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This site is an independent educational reference for amateur astronomy, focused on practical telescope use. We do not claim any institutional history or affiliation with past organizations.

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Our goal is to provide clear, reliable guidance for beginners and hobbyists. The telescope eyepiece magnification chart you are looking for is a core resource here. It explains how to calculate magnification using your telescope’s focal length and the eyepiece’s millimeter rating, and it offers recommended ranges for different objects.

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We present this information as a neutral, self-contained reference. While the domain has existed since at least 2003, we make no assertions about prior content or ownership. All current material is original to this independent project, intended solely for educational purposes.

Understanding the Telescope Eyepiece Magnification Chart: A Practical Guide for Beginner

When you first look through a telescope, the image you see is entirely shaped by two numbers: the telescope’s focal length and the eyepiece’s focal length. A magnification chart is simply a table that shows you what power you get when you pair different eyepieces with your specific telescope. This guide explains how to read, use, and avoid common mistakes with such a chart, so you can choose the right eyepiece for the Moon, planets, or deep-sky objects.

The Basic Formula Behind Every Chart

Every magnification chart is built on one equation: Magnification = Telescope Focal Length ÷ Eyepiece Focal Length. Both numbers are measured in millimeters (mm). For example, if your telescope has a focal length of 900 mm and you use a 10 mm eyepiece, the magnification is 90x (900 ÷ 10 = 90). If you switch to a 25 mm eyepiece, the magnification drops to 36x (900 ÷ 25 = 36). The chart simply lists these results for a range of eyepiece sizes, usually from 4 mm to 40 mm. You will also see a column for “exit pupil” (the diameter of the light beam leaving the eyepiece), which is calculated as Telescope Aperture ÷ Magnification. Exit pupil matters because it affects image brightness, especially for faint objects.

How to Build Your Own Chart (and What the Numbers Mean)

To create a personalized chart, you need three pieces of information from your telescope’s manual or the label on the tube: aperture (diameter of the main lens or mirror, in mm), focal length (in mm), and the maximum useful magnification. The maximum useful magnification is usually about 2x per millimeter of aperture. For a 100 mm telescope, that is roughly 200x. Beyond that, the image becomes dim and blurry because you are magnifying atmospheric turbulence, not detail. Your chart should list eyepiece focal lengths from low to high power. For a 900 mm telescope, a typical chart might look like this:

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• 40 mm eyepiece → 22.5x (very low power, wide field)
• 25 mm eyepiece → 36x (low power, good for star clusters)
• 15 mm eyepiece → 60x (medium power, good for the Moon)
• 10 mm eyepiece → 90x (high power, good for planets)
• 6 mm eyepiece → 150x (very high power, only on steady nights)
• 4 mm eyepiece → 225x (exceeds maximum useful magnification, avoid)

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Notice that the chart is not linear: each step down in eyepiece focal length increases magnification by a larger jump. This is why you should not buy eyepieces in a simple arithmetic sequence (e.g., 5, 10, 15, 20 mm). Instead, choose steps that roughly double or halve the magnification.

Decision Criteria: Which Magnification Should You Use?

The chart tells you what is possible, but not what is wise. Use these three criteria to pick a magnification for a given night:

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1. Object type – The Moon and planets tolerate high magnification (100x to 200x) because they are bright and small. Deep-sky objects like galaxies and nebulae are faint and large; they need low magnification (20x to 60x) to gather enough light and fit in the field of view. For example, the Andromeda Galaxy spans several full moons in the sky, so a 40 mm eyepiece (22.5x) is far better than a 6 mm eyepiece (150x).

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1. Atmospheric stability (seeing) – On a night when stars twinkle violently, high magnification will make the image boil. A good rule is to start at low power, then increase until the image starts to degrade, then back off one step. If the chart says 150x is possible, but the air is unsteady, 90x may be the practical limit.

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1. Your telescope’s aperture – A small 60 mm refractor has a maximum useful magnification of about 120x, so a 6 mm eyepiece on a 900 mm scope (150x) is already too much. A large 200 mm Dobsonian can handle 400x, but only on rare, perfect nights. Always check the chart against your aperture limit, not just the focal length.

Common Mistakes Beginners Make with Magnification Charts

The most frequent error is assuming that higher magnification is always better. This is false. High magnification shrinks the field of view, dims the image, and magnifies every wobble in the mount and every disturbance in the atmosphere. A 4 mm eyepiece on a cheap telescope often shows a blurry, dark circle instead of crisp detail. The second mistake is ignoring the exit pupil. If the exit pupil is larger than your eye’s pupil (about 7 mm in dark adaptation), you are wasting light. If it is smaller than 0.5 mm, the image becomes too dim and you see floating specks in your own eye. For example, a 900 mm telescope with a 40 mm eyepiece gives an exit pupil of 40 ÷ (900 ÷ aperture). For a 100 mm scope, that is 4.4 mm, which is fine. But a 4 mm eyepiece gives an exit pupil of 0.44 mm, which is borderline unusable.

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Another common mistake is buying a “2x Barlow lens” and then doubling every magnification on the chart without recalculating. A Barlow does double the effective focal length, so a 10 mm eyepiece becomes 5 mm equivalent. That is fine, but you must remember that the chart’s maximum useful magnification still applies. Also, many beginners forget that eyepiece quality matters more than focal length. A cheap 6 mm eyepiece with a narrow field of view may be worse than a good 10 mm eyepiece with a wide field. The chart gives you numbers, but it cannot tell you about eye relief (the distance your eye must be from the lens) or field distortion. Always read reviews or test eyepieces before buying.

A Compact Actionable Reference for Your First Night

Here is a quick, practical guide you can print and keep near your telescope. Assume a telescope with focal length F (in mm) and aperture A (in mm). Maximum useful magnification = 2 × A.

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• Low power (20x–40x): Use an eyepiece with focal length = F ÷ 30 (approximately). This is for star clusters, the Milky Way, and large nebulae. Example: F = 900 mm → 30 mm eyepiece.
• Medium power (50x–80x): Use an eyepiece with focal length = F ÷ 60. This is for the Moon, Jupiter’s moons, and Saturn’s rings. Example: F = 900 mm → 15 mm eyepiece.
• High power (100x–150x): Use an eyepiece with focal length = F ÷ 120. This is for lunar craters and planetary detail on steady nights. Example: F = 900 mm → 7.5 mm eyepiece (use a 7 mm or 8 mm).
• Very high power (above 150x): Only attempt if your aperture is at least 100 mm and the air is perfectly still. Use a Barlow lens with your medium eyepiece rather than buying a tiny 4 mm eyepiece, which often has poor eye relief.

Final Checks Before You Buy or Use an Eyepiece

First, verify your telescope’s focal length and aperture from the manufacturer’s label—do not guess. Second, calculate the exit pupil for any eyepiece you consider: Exit pupil = A ÷ Magnification. Keep it between 0.5 mm and 7 mm. Third, remember that magnification is not a measure of quality; a 60x view through a good eyepiece will show more than a 200x view through a poor one. Finally, use the chart as a starting point, not a rule. On a humid, turbulent night, drop one power level. On a cold, clear winter night, you may push one level higher. The chart is your map, but your eye is the final judge.