Aperture, Magnification & First Light
Telescope Eyepieces: Which Ones You Actually Need
The two eyepieces in the box are usually the least considered items in it. Choosing eyepieces is the cheapest real upgrade available, and it is mostly arithmetic.
By Scooter M. · Published · How we pick
An eyepiece does three things: it sets the magnification, it sets how wide a field you see, and it sets how bright the image is. Eyepiece choice is magnification, which is why this page sits in the basics hub rather than in an accessories section.
Focal length is the number that matters
magnification = telescope focal length / eyepiece focal length
On a 650mm telescope: 32mm gives 20x, 25mm gives 26x, 13mm gives 50x, 10mm gives 65x, 6mm gives 108x.
A shorter eyepiece gives more magnification. On a 1200mm Dobsonian those same five eyepieces give 38x, 48x, 92x, 120x and 200x, so the right eyepiece set depends entirely on the telescope. Run yours.
Exit pupil, and why low power looks brighter
The exit pupil is the width of the beam of light leaving the eyepiece. If it is wider than your own pupil, some of the light your telescope collected never enters your eye at all.
exit pupil in millimeters = aperture / magnification
A 200mm telescope at 40x: 200 / 40 = 5mm. The same telescope at 200x: 200 / 200 = 1mm.
A dark-adapted human pupil opens to about 7mm, less with age. An exit pupil above that wastes light; one below about 0.5mm gives an image too dim to be useful.
This is the arithmetic behind two familiar experiences. Low power looks bright because the exit pupil is large, and very high power looks dim because it is tiny. It is also why deep-sky observers reach for long eyepieces and planetary observers for short ones.
Which focal lengths to own
| Telescope | Wide field | General | High power |
|---|---|---|---|
| 400mm f/5.7 | 32mm = 13x | 13mm = 31x | 6mm = 67x |
| 650mm f/5 | 32mm = 20x | 13mm = 50x | 6mm = 108x |
| 750mm f/5 | 32mm = 23x | 13mm = 58x | 6mm = 125x |
| 1000mm f/9 | 32mm = 31x | 17mm = 59x | 8mm = 125x |
| 1200mm f/5.9 | 32mm = 38x | 17mm = 71x | 8mm = 150x |
| 1500mm f/10 | 32mm = 47x | 17mm = 88x | 10mm = 150x |
Barrel sizes, so you buy the right fitting
- 1.25 inch. The standard. Every telescope on this site accepts it, and almost every eyepiece sold is this size.
- 2 inch. Found on larger Dobsonians and better focusers. It allows genuinely wide low-power fields that 1.25-inch eyepieces physically cannot. Most 2-inch focusers include a 1.25-inch adapter.
- 0.965 inch. Obsolete, and found only on old or very cheap telescopes. If a telescope takes only these, that is a reason not to buy it.
What to buy first
- A moon filter, if the telescope is over about 100mm. The cheapest item here and the one that changes a session immediately.
- A long eyepiece, around 32mm, for wide-field finding and deep-sky views. No bundled eyepiece matches one, and it is the eyepiece that gets used most.
- A mid-range eyepiece, giving roughly one times the aperture in millimeters. This is the planetary workhorse.
- A 2x Barlow, or a short eyepiece, but not both. They cover the same ground. A Barlow is more flexible; a dedicated short eyepiece usually performs slightly better.
How this is funded: the buy buttons below are Amazon Associates links, and a qualifying purchase earns us a commission at no extra cost to you. It cannot move the ranking, because the order here is set by aperture per dollar, and no commission rate changes how much light a mirror collects. Full disclosure.
| # | Telescope | Best for | Aperture | Price |
|---|---|---|---|---|
| 1 | No image Celestron Eyepiece, Barlow and Filter Kit (1.25")Top pickFive Plossl eyepieces from 32mm down to 6mm plus a 2x Barlow. This is the accessory that actually changes what you see. | The upgrade that changes the most | 1.25" barrel64x useful | Check price#ad |
| 2 | No image Celestron AstroMaster Accessory Kit (1.25")Two extra eyepieces, a 2x Barlow and filters. The cut-down version of the full kit, for a smaller telescope. | A smaller first upgrade | 1.25" barrel64x useful | Check price#ad |
| 3 | No image Celestron Moon Filter (1.25")A neutral-density filter that screws into an eyepiece and makes the full Moon comfortable to look at. | The cheapest real upgrade | 1.25" thread64x useful | Check price#ad |
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Top pick · The upgrade that changes the most
Celestron Eyepiece, Barlow and Filter Kit (1.25")
Five Plossl eyepieces from 32mm down to 6mm plus a 2x Barlow. This is the accessory that actually changes what you see.
What the aperture allows
- Maximum useful magnification
- 64x
- 2 × 31.75mm of aperture
- Resolving limit (Dawes)
- 3.65 arcseconds
- 116 ÷ 31.75mm
- Cassini division (0.7″)
- Out of reach
- this aperture cannot resolve it
Magnification is telescope focal length divided by eyepiece focal length, so this kit is the variable in that equation. On a 650mm f/5 telescope the 32mm gives 20x, the 13mm gives 50x and the 6mm gives 108x. On a 1200mm Dobsonian the same three give 38x, 92x and 200x.
The 2x Barlow halves every eyepiece's effective focal length, which is where the ten-eyepieces-in-one claim comes from. It is true arithmetic, but a Barlow on a 6mm eyepiece produces 3mm, and on most beginner telescopes 3mm is beyond the useful magnification the aperture supports.
The useful lesson is that the two eyepieces in the telescope's box are usually the two least considered items in it. A 32mm Plossl in a fast Dobsonian gives a genuinely wide, bright field that no bundled eyepiece will match.
A moon filter is not optional at full Moon in anything over about 100mm. The Moon at 130mm is uncomfortably bright, and that is not a figure of speech.
What it does well
- Covers the whole useful magnification range in one purchase rather than five
- Plossl is a well-corrected classic design at a price where the alternatives are not
- The moon filter alone earns its place in anything over 100mm
What it costs you
- The shortest eyepieces exceed the useful magnification of a small telescope
- Plossl eye relief at 6mm is tight, which is awkward for spectacle wearers
- Several of the colored planetary filters see very little use
Skip this one if: Skip it if your telescope has a 2-inch focuser and you would rather put the money toward one good wide-field 2-inch eyepiece.
| Optical design | Five 1.25-inch four-element Plossl eyepieces, a 2x Barlow and seven filtersSource: Celestron Eyepiece, Barlow and Filter Kit — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
|---|---|
| Aperture | 1.25" barrelThe fitting size, not a light-collecting aperture. Check your focuser takes 1.25-inch eyepieces.Source: Celestron Eyepiece, Barlow and Filter Kit — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
| Eyepieces supplied | 32mm, 17mm, 13mm, 8mm and 6mm Plossl, each with a 52-degree apparent fieldSource: Celestron Eyepiece, Barlow and Filter Kit — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
| Barlow | 2x, which doubles the magnification of every eyepiece in the caseSource: Celestron Eyepiece, Barlow and Filter Kit — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
| Filters | A moon filter plus the common colored planetary filtersSource: Celestron Eyepiece, Barlow and Filter Kit — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
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Pick 2 · A smaller first upgrade
Celestron AstroMaster Accessory Kit (1.25")
Two extra eyepieces, a 2x Barlow and filters. The cut-down version of the full kit, for a smaller telescope.
What the aperture allows
- Maximum useful magnification
- 64x
- 2 × 31.75mm of aperture
- Resolving limit (Dawes)
- 3.65 arcseconds
- 116 ÷ 31.75mm
- Cassini division (0.7″)
- Out of reach
- this aperture cannot resolve it
This is the sensible kit for a 70mm to 114mm telescope, where the five-eyepiece set's shortest focal lengths would exceed the useful magnification the aperture supports anyway.
On a 700mm f/10 refractor, a 2x Barlow on a 10mm eyepiece gives 140x, which is exactly the 2x-aperture ceiling for a 70mm objective. There is nothing left to gain past that, and a kit that stops there is a kit sized to the telescope.
The filters are the same colored planetary set as the larger kit, and they see the same modest amount of use.
What it does well
- Sized sensibly for a small telescope rather than overshooting its aperture
- Cheaper than the five-eyepiece case
- Includes the moon filter, which is the piece that matters most
What it costs you
- Fewer eyepieces, so bigger jumps between magnifications
- Little point if the telescope is already 130mm or larger
Skip this one if: Skip it if your telescope is 130mm or bigger. Buy the five-eyepiece kit instead; you have the aperture to use it.
| Optical design | Two 1.25-inch eyepieces, a 2x Barlow lens and a set of filtersSource: Celestron AstroMaster Accessory Kit — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
|---|---|
| Aperture | 1.25" barrelThe fitting size, not a light-collecting aperture.Source: Celestron AstroMaster Accessory Kit — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
| Barlow | 2xSource: Celestron AstroMaster Accessory Kit — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
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Pick 3 · The cheapest real upgrade
Celestron Moon Filter (1.25")
A neutral-density filter that screws into an eyepiece and makes the full Moon comfortable to look at.
What the aperture allows
- Maximum useful magnification
- 64x
- 2 × 31.75mm of aperture
- Resolving limit (Dawes)
- 3.65 arcseconds
- 116 ÷ 31.75mm
- Cassini division (0.7″)
- Out of reach
- this aperture cannot resolve it
The full Moon through a 130mm telescope is genuinely too bright to look at comfortably, and the discomfort costs you detail: a dazzled eye resolves less.
The filter is neutral density, meaning it cuts the light without shifting the color. Contrast goes up, the glare goes away, and the terminator, the line between lunar day and night where the shadows are longest, becomes the best thing in the sky.
It is the single cheapest item that changes an observing session, and it is the one accessory almost every beginner buys second rather than first.
What it does well
- Turns the full Moon from uncomfortable into detailed
- Threads into any standard 1.25-inch eyepiece
- Costs less than almost anything else that improves a view
What it costs you
- Only useful on the Moon and on Venus
- Redundant if you buy an accessory kit, which usually includes one
Skip this one if: Skip it if you are already buying an eyepiece and filter kit. There is one in the case.
| Optical design | Neutral-density filter that threads into a 1.25-inch eyepiece barrelSource: Celestron Moon Filter — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
|---|---|
| Aperture | 1.25" threadThe fitting size. It threads into the eyepiece, not into the telescope.Source: Celestron Moon Filter — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
| Effect | Reduces lunar glare, improves contrast and reveals surface detailCelestron's own description of what the filter does.Source: Celestron Moon Filter — the brand's own listing on Amazon.com (retrieved September 1, 2026) |
What not to buy
- Cheap zoom eyepieces. The zoom mechanism costs sharpness and field width across the whole range, and at this price the compromise is severe.
- A 3x Barlow. Even a 2x pushes most beginner telescopes to their ceiling. A 3x guarantees empty magnification.
- Colored planetary filters, at first. Most see two uses. The moon filter is the one that earns a place in the case.
- A very short eyepiece for a small telescope. A 4mm eyepiece in a 700mm f/10 refractor gives 175x, which is above the 140x its 70mm aperture supports.
Questions people actually ask
What eyepiece focal lengths do I need?
Three: a long one around 32mm for wide-field finding, a mid-range one giving roughly one times the aperture in millimeters, and a short one at or just under two times the aperture. On a 650mm f/5 telescope that is roughly 32mm, 13mm and 6mm.
What does eyepiece focal length mean?
It is the number printed on the eyepiece in millimeters, and it sets magnification: the telescope's focal length divided by the eyepiece's. A shorter eyepiece gives higher magnification and a narrower, dimmer field.
What is exit pupil?
The width of the light beam leaving the eyepiece, equal to the aperture divided by the magnification. A dark-adapted human pupil opens to about 7mm, so an exit pupil wider than that wastes some of the light the telescope collected.
Are expensive eyepieces worth it?
Up to a point. Moving from bundled eyepieces to decent Plossls is a clear improvement. Beyond that, premium wide-field eyepieces mostly buy a larger apparent field and better edge sharpness, which matters more on fast telescopes than on slow ones.
What size eyepiece fits my telescope?
Almost certainly 1.25 inch, which is the standard barrel size. Larger Dobsonians and better focusers also take 2-inch eyepieces, usually with a 1.25-inch adapter included. The obsolete 0.965-inch size appears only on old or very cheap telescopes.
Do I need a Barlow lens?
It is a reasonable substitute for buying a second short eyepiece, since a 2x Barlow doubles the magnification of everything in your case. Avoid 3x Barlows: they push most beginner telescopes well past their useful ceiling.
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