Three numbers come out of an eyepiece and only one of them is on the box
Magnification is the telescope focal length divided by the eyepiece focal length. A 1200 mm telescope with a 9 mm eyepiece gives 133x. That much everyone knows, and it is the least interesting of the three.
True field of view is roughly the apparent field of the eyepiece divided by that magnification. The apparent field is a property of the eyepiece design — how wide the window looks when you put your eye to it — and it ranges from around 40 degrees on simple designs to past 100 on the extreme ones. A 52-degree eyepiece at 133x shows about 0.39 degrees of sky, which is 23 arcminutes, which is not quite enough to hold the full moon.
Exit pupil is the diameter of the cone of light leaving the eye lens, and it equals the aperture divided by the magnification. It also equals the eyepiece focal length divided by the focal ratio, which is the more useful form because it means the focal ratio alone decides which eyepieces make sense on a given telescope. At f/6, a 32 mm eyepiece gives a 5.3 mm exit pupil on any f/6 telescope regardless of size.
Why the exit pupil is the number that decides things
Your eye has a pupil, and in full dark adaptation it opens to somewhere between about 5 mm and 8 mm depending mostly on your age. If the beam arriving is wider than that opening, the extra light lands on the iris. The telescope is then working at a reduced effective aperture equal to the true aperture scaled by the ratio of your pupil to the exit pupil, and the light lost goes as the square of that ratio.
| Exit pupil | Into a 6 mm pupil | What it feels like |
|---|---|---|
| 7 mm | 26% of the light missed | Bright and wide, but a 200 mm scope is behaving like a 171 mm one |
| 5 mm | Nothing lost | The classic rich-field setting for faint extended objects |
| 2 mm | Nothing lost | General purpose, holds up in mediocre seeing |
| 0.7 mm | Nothing lost | High power on planets and doubles, dim, floaters start showing |
| 0.4 mm | Nothing lost | Empty magnification territory for most apertures and most nights |
Both ends are personal. The wide end is set by your own pupil, which is worth measuring rather than assuming — take a photograph of your eye in the dark with a flash and a ruler in the frame, or have someone measure it, and use your own figure in the calculator. The narrow end is set by when the view stops being pleasant for you, which is a mix of the seeing that night, floaters in your own vitreous and your tolerance for a dim image.
A barlow is a focal length change, not an eyepiece change
A barlow multiplies the effective focal length of the telescope, which multiplies every magnification and divides every exit pupil and every true field. It does not change the aperture, so a 2x barlow on an f/6 telescope makes it behave as f/12 for the purposes of this arithmetic, and a 20 mm eyepiece behind it gives exactly the exit pupil that a 10 mm eyepiece would give without it. That equivalence is real optically, which is why a barlow plus two eyepieces covers more of the useful range than four eyepieces bought separately, and it is the cheapest way to fill gaps in a case.
The catch is eye relief and comfort rather than optics. The 20 mm behind a barlow keeps the eye relief of a 20 mm, which for spectacle wearers is the whole argument. Against that, the barlow adds glass, adds a joint that can flex, and pushes the focus point outward, which some focusers cannot reach. The reducer and barlow page covers what the spacing does to the actual factor, because 2x is only 2x at one particular distance.
Field of view, drift, and why the crossing time matters
On a mount with no drive, the object crosses the field at the sidereal rate scaled by the cosine of its declination. At the celestial equator that is 15.04 arcseconds a second, so a field 23 arcminutes wide takes about 93 seconds to cross end to end, and roughly half that from centre to edge. At declination 60 the cosine halves it, so the same field takes over three minutes. Near the pole the object barely drifts at all — it rotates about the field instead.
That is why the crossing time is a practical number rather than a curiosity. It tells you how often you will be nudging the tube, and it is the reason a wide-field eyepiece is worth more on an undriven mount than a narrow one of the same magnification. It also sets a floor under what you can do at high power without a drive: at 300x through a 50-degree eyepiece, the field is 10 arcminutes and an equatorial object crosses it in 40 seconds.
What this page does not tell you
It does not tell you the highest magnification worth using. Every rule of thumb quoted for that — so many times the aperture in inches, or in millimetres — is a convention rather than a measurement, and the real ceiling on any given night is the atmosphere, which changes hourly. The honest version is the pair of exit pupil bounds you enter yourself: the widest that your pupil accepts, and the tightest that still gives you a view you enjoy. Everything between those is usable and the top end of it is usable only sometimes.
It also assumes the apparent field figure you enter is right. Those figures come from eyepiece makers and are not measured to a common standard, so two eyepieces both marked 68 degrees can differ by several degrees of real field. If precision matters, use the field stop diameter instead — divide it by the telescope focal length and multiply by 57.3 to get degrees of true field directly, with no apparent-field number involved.
Questions people ask
What magnification does my telescope give?
The telescope focal length divided by the eyepiece focal length, with any barlow or reducer factor applied to the telescope side first. A 1200 mm telescope with a 9 mm eyepiece gives 133x, and putting a 2x barlow in front of the same eyepiece gives 267x. Note that the aperture never enters this calculation, which is why two telescopes of very different size can give identical magnification with the same eyepiece and look nothing alike doing it.
What is exit pupil and why does it matter more than magnification?
It is the width of the light beam leaving the eyepiece, equal to the aperture divided by the magnification, or equivalently the eyepiece focal length divided by the focal ratio. It matters because your own eye pupil is a hard aperture stop on the system. A beam wider than your pupil has its outer ring blocked by your iris, and the telescope then performs as though it had a smaller objective. Past that point, a longer eyepiece buys you a wider field but not a brighter one.
How do I find my own dark-adapted pupil size?
Measure it rather than looking up an average, because the spread between people of the same age is wide. A phone photograph taken in a dark room with a ruler held beside your eye, with the flash providing the only light, gives a workable figure once you scale off the ruler. The number shrinks over a lifetime, typically ending up well under what it was at twenty, and that shift is the reason older observers often find long focal length eyepieces less useful than the reviews suggest.
Is true field of view just apparent field divided by magnification?
That is the approximation this page uses and it is good to a few percent for most eyepieces. It runs slightly optimistic on very wide designs because those eyepieces distort the field toward the edge, so the sky actually shown is a little less than the simple division suggests. The exact method uses the field stop diameter in millimetres: divide it by the telescope focal length and multiply by 57.3 for degrees. Where the eyepiece maker publishes a field stop figure, that route is the better one.
Does a barlow reduce image quality?
It adds glass and a mechanical joint, so it can, but the size of the effect depends entirely on the barlow and how square everything sits in the focuser. What is certain is the arithmetic: it multiplies the effective focal length, so magnification goes up by the factor, exit pupil and true field go down by it, and the focal ratio goes up by it. A 20 mm eyepiece behind a 2x barlow gives the same magnification and exit pupil as a 10 mm alone while keeping the longer eye relief, which for spectacle wearers is usually the deciding argument.