Eyepiece Focal Length to Magnification Chart
Magnification equals telescope focal length divided by eyepiece focal length, so a 10mm eyepiece in a common 1200mm telescope gives 120x, while the same 10mm eyepiece in a 650mm telescope gives only 65x. The eyepiece number alone never tells you the magnification; the telescope it is inserted into does.
An eyepiece's focal length is not its magnification. The same eyepiece produces a different magnification in every telescope it is used in, because magnification is telescope focal length divided by eyepiece focal length. A 10mm eyepiece gives 65x in a 650mm telescope and 203x in a 2032mm telescope. This chart converts the fourteen most common eyepiece focal lengths sold today into magnification across seven common telescope focal lengths, several tied to real telescope families, and adds exit pupil, the number that decides whether that magnification will actually look good.
How do you calculate eyepiece magnification?
Magnification equals telescope focal length divided by eyepiece focal length, both measured in millimeters. Focal length is printed on almost every telescope's spec sheet or stamped on the tube itself, and eyepiece focal length is printed on the eyepiece barrel. Nothing else enters the calculation: not the eyepiece's apparent field of view, not its brand, not its price.
A useful shortcut worth memorizing: halving the eyepiece focal length doubles the magnification, and halving the telescope focal length halves it. A 12.5mm eyepiece gives exactly twice the magnification of a 25mm eyepiece in the same telescope, every time, in every telescope.
What magnification does each eyepiece give on my telescope?
The table below runs the calculation for fourteen common eyepiece focal lengths across seven telescope focal lengths, from a short 400mm rich-field refractor up to a 2032mm Schmidt Cassegrain. Find the row for your eyepiece and the column closest to your telescope's focal length.
| Eyepiece (mm) | 400mm | 650mm | 750mm | 900mm | 1200mm | 1500mm | 2032mm |
|---|---|---|---|---|---|---|---|
| 4 | 100x | 163x | 188x | 225x | 300x | 375x | 508x |
| 5 | 80x | 130x | 150x | 180x | 240x | 300x | 406x |
| 6 | 67x | 108x | 125x | 150x | 200x | 250x | 339x |
| 7 | 57x | 93x | 107x | 129x | 171x | 214x | 290x |
| 8 | 50x | 81x | 94x | 113x | 150x | 188x | 254x |
| 9 | 44x | 72x | 83x | 100x | 133x | 167x | 226x |
| 10 | 40x | 65x | 75x | 90x | 120x | 150x | 203x |
| 12.5 | 32x | 52x | 60x | 72x | 96x | 120x | 163x |
| 15 | 27x | 43x | 50x | 60x | 80x | 100x | 135x |
| 17 | 24x | 38x | 44x | 53x | 71x | 88x | 120x |
| 20 | 20x | 33x | 38x | 45x | 60x | 75x | 102x |
| 25 | 16x | 26x | 30x | 36x | 48x | 60x | 81x |
| 32 | 13x | 20x | 23x | 28x | 38x | 47x | 64x |
| 40 | 10x | 16x | 19x | 23x | 30x | 38x | 51x |
Column reference: 650mm matches the Heritage 130P and StarSense DX130 family, 900mm matches the StarSense Explorer LT 80AZ, 1200mm matches the Classic 200 Dobsonian and StarSense Dob 8, and 2032mm matches the NexStar 8SE and Evolution 8. If your telescope's focal length falls between two columns, interpolate, or run it exactly through the magnification calculator.
What is exit pupil and why does it matter as much as magnification?
Exit pupil is the diameter, in millimeters, of the beam of light leaving the eyepiece, calculated as eyepiece focal length divided by telescope focal ratio. It determines how bright and how comfortable the view feels independent of magnification. A dark adapted adult pupil opens to roughly 7mm at best and shrinks toward 5mm with age, so an exit pupil larger than that wastes light the eye cannot use. Below about 0.5mm the image dims, softens and starts to shake with every small vibration, since you are viewing through a genuinely tiny beam of light.
| Eyepiece (mm) | f/4 | f/5 | f/6 | f/8 | f/10 | f/12 |
|---|---|---|---|---|---|---|
| 4 | 1 | 0.8 | 0.67 | 0.5 | 0.4 | 0.33 |
| 5 | 1.25 | 1 | 0.83 | 0.63 | 0.5 | 0.42 |
| 6 | 1.5 | 1.2 | 1 | 0.75 | 0.6 | 0.5 |
| 7 | 1.75 | 1.4 | 1.17 | 0.88 | 0.7 | 0.58 |
| 8 | 2 | 1.6 | 1.33 | 1 | 0.8 | 0.67 |
| 9 | 2.25 | 1.8 | 1.5 | 1.13 | 0.9 | 0.75 |
| 10 | 2.5 | 2 | 1.67 | 1.25 | 1 | 0.83 |
| 12.5 | 3.13 | 2.5 | 2.08 | 1.56 | 1.25 | 1.04 |
| 15 | 3.75 | 3 | 2.5 | 1.88 | 1.5 | 1.25 |
| 17 | 4.25 | 3.4 | 2.83 | 2.13 | 1.7 | 1.42 |
| 20 | 5 | 4 | 3.33 | 2.5 | 2 | 1.67 |
| 25 | 6.25 | 5 | 4.17 | 3.13 | 2.5 | 2.08 |
| 32 | 8 | 6.4 | 5.33 | 4 | 3.2 | 2.67 |
| 40 | 10 | 8 | 6.67 | 5 | 4 | 3.33 |
Read this table against the aperture chart's sweet spot: most observers are happiest between about 0.7mm and 4mm of exit pupil. Under roughly 0.7mm the image gets noticeably dim on deep sky targets. Above roughly 4mm you are firmly in low-power finder territory, wide and bright but with little magnification.
How many eyepieces do you actually need?
Three, arranged around specific jobs rather than around round numbers. The pattern that works for nearly every telescope:
- A low power finder eyepiece, typically 25mm to 32mm, giving the widest true field and the brightest, most forgiving view for locating a target before increasing power. A 32mm Plossl is the standard choice here because a 52 degree apparent field is close to the physical limit a 1.25 inch barrel can deliver.
- A workhorse mid-power eyepiece, typically 9mm to 15mm, for the majority of an observing session: brighter deep sky objects, lunar detail, and planets on nights of average seeing. The Celestron X-Cel LX 9mm earns its place here on eye relief alone, which matters most at the focal lengths people actually observe through for hours.
- A high power planetary eyepiece, typically 5mm to 7mm, reserved for the best nights of seeing on the Moon, planets and tight double stars. A budget 6mm wide angle eyepiece covers this affordably; it will sit unused on nights of poor seeing, and that is correct behavior, not a wasted purchase.
Everything past those three is refinement, not necessity. A telescope shipped with a stock Kellner or Plossl pair almost always already covers the low and mid slots adequately for a first year of observing, which means the single most cost-effective first eyepiece purchase for most beginners is the high-power planetary eyepiece the stock kit is missing.
What's the difference between a zoom eyepiece and a set of fixed eyepieces?
A zoom eyepiece, such as the SVBONY SV135 7mm to 21mm zoom , covers most of the useful magnification range without ever removing the eyepiece from the focuser. That single fact matters more than it sounds: swapping eyepieces in the dark is the single most common way a beginner loses a target they had just found, and a zoom removes that failure point entirely. The tradeoff is apparent field of view, which narrows noticeably at the long end, typically down to around 40 degrees, versus 50 to 60 degrees on an equivalent fixed eyepiece.
At the expert end, a premium zoom like the Baader Hyperion Mark IV 8mm to 24mm zoom holds a genuinely wide 68 degree field at its short setting and stays sharp across the whole range, at a price close to four decent fixed eyepieces combined. It earns that price only for observers who go out often enough that the convenience of never swapping glass pays for itself across many nights, not for someone who observes a few times a season.
A fixed set, by contrast, is optically simpler at every focal length and typically cheaper per eyepiece, at the cost of needing to physically swap glass, ideally with a red light nearby, each time you change magnification. For an undriven Dobsonian, a genuinely wide fixed eyepiece such as the Explore Scientific 82 degree 18mm also buys something a zoom rarely matches at that focal length: enough true field that a target drifts across the view for much longer before you need to nudge the tube.
How does eyepiece choice interact with the aperture ceiling?
Every row in the magnification table above should be checked against your telescope's own maximum useful magnification before buying. A 4mm eyepiece giving 508x in a 2032mm telescope sounds appealing, but that number only means something if the telescope's aperture, in this case a common 203mm Schmidt Cassegrain, actually supports 508x. It does not: 203mm caps out around 406x by the 2x per millimeter rule, and the atmosphere caps it further to roughly 200x to 250x on a typical night. See the full breakdown in maximum useful magnification by aperture before buying anything shorter than about 5mm.
For a telescope-specific answer rather than the representative focal lengths used here, run your exact numbers through the magnification calculator or the eyepiece calculator, both of which also compute exit pupil and true field of view for your specific eyepiece and telescope pairing rather than the nearest column on a chart.
Related reading
Frequently asked questions
How do you calculate eyepiece magnification?
Divide your telescope focal length by the eyepiece focal length, both in millimeters. A 1200mm telescope with a 10mm eyepiece gives 120x. The eyepiece never has a fixed magnification on its own, it only has a fixed magnification once you know which telescope it is inserted into, which is why buying eyepieces by focal length rather than by an advertised power figure is the correct approach.
What magnification is too high for my eyepiece?
Whatever pushes past your aperture ceiling of roughly 2x per millimeter. A 6mm eyepiece in a 650mm f/5 telescope gives 108x, well inside the 130mm aperture ceiling of 260x. That same 6mm eyepiece in a 2032mm telescope gives 339x, past the aperture ceiling for anything under about 170mm. Check the chart against your specific aperture, not just the eyepiece number.
What is exit pupil and why does it matter?
The diameter of the beam of light leaving the eyepiece, in millimeters, calculated as eyepiece focal length divided by focal ratio. Below about 0.5mm the image looks dim and mushy. Above about 7mm the light is wasted because an adult dark-adapted pupil rarely opens wider than that, and it shrinks further with age, so nothing above 7mm is actually usable regardless of the math.
Do I need a whole set of eyepieces or just one zoom?
A fixed 3-eyepiece kit, low, mid and high power, gives the sharpest view at each setting and is usually cheaper overall. A zoom eyepiece trades some sharpness and field of view at the edges for the convenience of never swapping glass in the dark, which matters most to beginners still learning to find targets. Both are legitimate choices; the zoom is the more forgiving one to start with.
Why does the same eyepiece give different magnification on different telescopes?
Because magnification depends on telescope focal length, not eyepiece focal length alone. A 25mm eyepiece gives 26x in a 650mm telescope and 81x in a 2032mm telescope. This is also why a used eyepiece is always a good buy across telescope brands, since it is the pairing with your specific focal length that determines the result, not the eyepiece brand or the telescope brand.
What eyepiece should I buy first if I only buy one?
A mid-power eyepiece somewhere around 9mm to 12mm in a typical short-tube reflector, which gives a comfortable, sharp, all-purpose view of the Moon, planets and brighter deep sky objects without demanding perfect seeing or perfect tracking. Buy the low-power wide field eyepiece second, since most telescopes already ship with something usable at that end, and a high-power planetary eyepiece third.
How we choose: we compare published manufacturer specifications, optical figures we can verify, and reviews from owners who have used the equipment under real skies. We do not test gear in person. Never point any telescope, finder or binocular at the Sun without a certified full-aperture solar filter fitted over the front of the instrument.
Recording your own eyepieces, exit pupils and sessions? The Observing & Astrophotography Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.