Telescope eyepiece calculator

What the number on an eyepiece means

An eyepiece is labelled with one number: its own focal length, in millimetres. A 25mm eyepiece, a 10mm eyepiece, a 6mm eyepiece. The confusing part is that the scale runs backwards from what most people expect — the smaller number gives the higher magnification, because magnification is a division:

magnification = telescope focal length ÷ eyepiece focal length

An eyepiece therefore has no magnification of its own. The same 10mm gives 65× in a 650mm telescope and 203× in a 2032mm one — a threefold difference from the identical piece of glass. This is why “which eyepiece should I buy” has no universal answer, and why a list of recommended focal lengths that does not ask what telescope you own is guessing.

The second number that matters is your telescope’s focal ratio: focal length divided by aperture, written f/5, f/8.9, f/10. That single figure is what converts an eyepiece focal length into a view, and it is the number the calculator above is really working from. Two telescopes of the same focal ratio behave identically as far as eyepieces are concerned, whatever their size — a 130mm f/5 and a 305mm f/5 want the same focal lengths in the focuser. The bigger one just shows more with them.

Work backwards from the exit pupil

Hold an eyepiece up to a lit sky at arm’s length and you will see a small bright disc floating just above the lens. That disc is the exit pupil: the width of the beam of light leaving the telescope and entering your eye. It has a formula as simple as magnification’s, and it is far more useful:

exit pupil = eyepiece focal length ÷ focal ratio

The reason to care is that your own eye has an aperture too. A dark-adapted pupil opens to about 7mm in a young adult and rather less — typically five to six millimetres — from middle age onwards. If the exit pupil is wider than your pupil, the surplus light lands on your iris. The view does not get brighter. You have paid for aperture you are then throwing away at the last centimetre.

Now rearrange it. That is the whole trick, and it is the step most eyepiece advice skips:

eyepiece focal length = target exit pupil × focal ratio

Decide what the beam of light should look like, and the eyepiece to buy falls out of the arithmetic. Roughly five to six millimetres is the low-power, wide-field band — as much sky and as much light as your eye can physically accept. Two to three millimetres is the general-purpose band, where the sky background is dark enough to give contrast but faint objects are still bright: this is the eyepiece that will spend most of its life in the focuser. Around one millimetre is the planetary band, and below about 0.7mm the image is usually a soft, dim disappointment on any ordinary night.

Two rules that are usually taught separately turn out to be the same rule. At a 1mm exit pupil, the magnification is numerically equal to the aperture in millimetres — a 203mm telescope is at 203×. At a 0.5mm exit pupil you are at exactly 2× per millimetre of aperture, which is the conservative ceiling on useful magnification. So “never go below a half-millimetre exit pupil” and “never exceed fifty times per inch” are two ways of saying one thing, and you only have to remember whichever you find easier.

The practical consequence is that focal ratio, not aperture, decides which eyepieces you should own. A fast f/5 telescope reaches a 2.5mm exit pupil with a 12mm eyepiece; a slow f/10 needs a 25mm for the same view. Move an eyepiece collection between the two and everything shifts a full band. That is also why a 40mm eyepiece — a size sold everywhere and recommended constantly — is close to useless in a fast scope: at f/5 it produces an 8mm exit pupil, wider than any human pupil, so a measurable fraction of the light your mirror collected never enters your eye at all. In an f/10 telescope the same 40mm gives a perfectly sensible 4mm. Neither eyepiece is better. The scopes are different.

Apparent field and true field are different numbers

Eyepieces are also sold with a degree figure — 50°, 68°, 82°, sometimes 100°. That is the apparent field of view: how wide the circle of light appears to your eye, like the difference between watching a film on a laptop and in a cinema. It is a property of the eyepiece design alone and has nothing to do with your telescope.

What you actually want to know is the true field of view: how much real sky is inside that circle. The two are linked by the magnification.

true field = apparent field ÷ magnification

The full Moon is about half a degree across, which makes a convenient ruler. A true field of 1° holds two Moons side by side; 0.25° frames half of one. Switch the apparent field control in the calculator between 50° and 82° and watch what happens: the magnification column does not move at all, and the true field column grows by roughly two-thirds. A wide-angle eyepiece does not magnify more. It shows you more sky at the same magnification, which is a genuinely different and often better thing — a target drifts out of an unguided view more slowly, and finding something is much easier when the field is wide.

It is worth being clear about what the extra degrees cost. Wide-field designs use more glass elements, weigh more, and are dramatically more expensive at the same focal length. A 68° eyepiece is a real improvement over a 50° Plössl for sweeping and for low-power work. At high power, where you are looking at a planet occupying a tiny fraction of the field anyway, the extra apparent field buys you comfort rather than capability.

Two good eyepieces beat a set of seven

The default first accessory purchase is a boxed set: four or five eyepieces, a Barlow, a row of coloured filters, all in a moulded case, for less than the price of one decent eyepiece. It is an appealing object and it is usually the wrong buy, for reasons that follow directly from the table above.

The first is that a set is assembled to look complete rather than to suit your telescope. Sets are built around round numbers, and the calculator will normally flag two or three of them as either overfilling your pupil or sitting past your magnification ceiling. You are buying seven items to use four. The second is that adjacent focal lengths in a set are often so close together that the views are hard to tell apart; the useful steps are roughly factors of two, not increments of five millimetres.

The third is the coloured filters. Planetary colour filters were genuinely useful in the era of small refractors and film, and they still have niche uses, but for most beginners they sit in the case unused for years. The one filter that changes what a beginner sees is a neutral-density or polarising Moon filter, because an unfiltered full Moon in a 200mm telescope is uncomfortably bright.

The honest alternative is to spend the same money on two eyepieces you will use for twenty years: one in the general-purpose band and one in the planetary band, both from the calculator above. Keep the eyepiece your telescope came with as the third. Almost every telescope ships with a 25mm or thereabouts, which usually lands in the low-power band already, so the two you buy are the two you are actually missing.

There is one shape of set that survives this argument: a small number of fixed focal lengths, spaced by roughly a factor of two, in a wide-field design. A 6 / 9 / 15 / 20mm grouping at 68° lands in useful bands for anything from about f/4 to f/8, which covers most Newtonians and Dobsonians. It is a different product from a case of Plössls and filters, and the calculator will tell you within a few seconds whether those four sizes land where they should in your telescope. Check it against your own numbers before buying — in a slow f/10 scope the same four sizes bunch up at the high-power end and leave you with nothing wide.

1.25-inch and 2-inch: what the barrel changes

Eyepieces come in two barrel diameters, named in inches because the standard is American and old. The 1.25-inch barrel is the near-universal default; the 2-inch is a larger fitting found on bigger Dobsonians, most Schmidt-Cassegrains via an adapter, and better refractors. A very cheap or very old telescope may use a 0.965-inch barrel, which is obsolete and worth knowing about only so you can avoid buying eyepieces that will not fit.

The barrel does not change magnification, brightness, or sharpness. What it changes is the maximum true field, and it does so through a part you never see: the field stop, the ring inside the eyepiece that defines the edge of the view. A 1.25-inch barrel physically cannot contain a field stop wider than about 27mm, and that puts a hard ceiling on how much sky any 1.25-inch eyepiece can show, no matter what is printed on it.

This produces one of the most common wasted purchases in amateur astronomy. A 40mm 1.25-inch eyepiece and a 32mm 1.25-inch eyepiece at 50° deliver almost exactly the same true field, because both are already up against the same barrel limit. The 40mm gives you lower magnification, a wider exit pupil, and no additional sky — and in a fast telescope that wider exit pupil is the wasted light described above. In 1.25-inch fittings, a 32mm at 50° is effectively as wide as it gets. If you want genuinely more sky than that, the answer is a 2-inch focuser and a 2-inch eyepiece, not a longer 1.25-inch one.

The practical guidance is unglamorous. Buy 1.25-inch for everything from the medium band upwards, because at those focal lengths the barrel is not the constraint and 1.25-inch eyepieces are cheaper, lighter, and vastly more available. Consider 2-inch only for the low-power eyepiece, only if your focuser already takes it, and only after checking that your telescope’s focal ratio can put a sensible exit pupil at that focal length in the first place. A slow telescope cannot deliver a wide exit pupil at any barrel size, which is why 2-inch eyepieces do far less for an f/10 Schmidt-Cassegrain than for an f/5 Dobsonian.

Where a Barlow fits, and where it does not

A Barlow lens sits between the focuser and the eyepiece and multiplies the magnification, usually by two. Optically it behaves as though every eyepiece behind it has had its focal length halved: a 20mm behind a 2× Barlow performs as a 10mm, and the exit pupil halves with it. That means a Barlow genuinely doubles the size of a small collection, and it is the reason the spacing between eyepieces you buy should not be a factor of two if you intend to own one — you would only be duplicating what the Barlow already gives you.

What a Barlow cannot do is move your ceiling. If the calculator says your telescope runs out of useful magnification, no Barlow changes that; it will happily take you past the limit and the result is a larger, dimmer, softer image. Barlows also add two more air-to-glass surfaces, so a cheap one placed in front of a good eyepiece will cost you more than the extra magnification is worth. The compensating advantage is real, though: a longer eyepiece behind a Barlow keeps the longer eyepiece’s eye relief, which is far more comfortable than squinting into a bare 4mm.

Eye relief, and why short eyepieces are uncomfortable

Eye relief is the distance behind the eyepiece at which your eye has to sit to see the whole field. In simple designs it scales roughly with focal length, which is why a 25mm Plössl is pleasant and a 6mm one of the same design requires pressing your eyelashes against the glass. If you observe wearing spectacles — and you should, if you have astigmatism, because a telescope cannot correct it — you need somewhere around 15mm of eye relief to see the full field, and most short simple eyepieces do not provide it.

There are two ways out. Buy a design that holds its eye relief constant across focal lengths, which is what the more expensive planetary lines exist to do, or reach high magnification with a longer eyepiece behind a Barlow. This is the specification that most often decides whether someone keeps using a high-power eyepiece or quietly stops, and it is not one the arithmetic on this page can compute for you. It is published per eyepiece, and it is worth looking up before you order.

What this page cannot tell you

The calculator computes geometry, and geometry is only part of an eyepiece. It cannot tell you whether a particular eyepiece is sharp to the edge of its field, how much scattered light it throws around a bright planet, whether it fogs on a damp night, or whether the rubber eyecup will still be attached in three years. Those are questions about a specific piece of manufacturing, and we have not looked through any of them.

It also cannot tell you what your sky will allow. The high-power row assumes the atmosphere will hold still, and on most nights from most locations it will not. Observers routinely find that the eyepiece they reach for is a band lower than the one the arithmetic recommends, because a smaller, sharper image beats a larger, wobbling one. Treat the planetary figure as the best case rather than the expected case.

Aperture and focal length should be taken from your telescope’s specification table or the label on the tube, never from a product title — titles are marketing copy, and at least one listing in our own data states an aperture that its own detail table contradicts. Prices shown alongside any product here were captured on 2026-08-20 and will drift. For the judgements this method cannot make — how a given eyepiece actually performs in a given telescope — the eyepiece forums at Cloudy Nights are the best resource available, they pay us nothing, and they will tell you things we cannot.