Eyepiece Magnification Chart (4mm to 32mm)

A 25 mm eyepiece on a 1,200 mm telescope gives 48x. Swap in a 10 mm and you get 120x. A 4 mm reaches 300x.

Every one of those numbers is the same division. Telescope focal length, divided by eyepiece focal length.

The arithmetic is easy. The trap is the ceiling, because a power past it buys nothing at all.

On 203 mm (8 in) the ceiling is 2 × 203 = 406x. A printed number above that line is a promise the glass cannot keep.

Ten eyepiece focal lengths, from 4 mm to 32 mm. The chart sets them against six common telescope focal lengths, so you can find your tube.

Find your row and your column. The intersection is your answer.

One division fills the whole table. Telescope focal length ÷ eyepiece focal length.

Read it in three steps.

First, find the column for your telescope. Its focal length is the number on the tube, not the aperture.

Second, find the row for your eyepiece. Third, the cell where they meet is your power.

Worked example. A 1,200 mm telescope, a 25 mm eyepiece. 1,200 ÷ 25 = 48x.

Now read the rows you are most likely to be holding. On a 1,200 mm telescope a 6 mm eyepiece is 200x, and a 9 mm is 133x.

A 10 mm gives 120x. A 20 mm gives 60x, and a 25 mm gives 48x.

The same telescope. The same glass. Five very different views.

The rest of the ladder runs the other way. A 4 mm is 300x, an 8 mm is 150x, a 13 mm is 92x, a 17 mm is 71x, and a 32 mm is 38x.

Moving down the rows drops the power, and it widens the field every time.

Move across the columns and the same eyepiece changes again. A 25 mm gives 26x on 660 mm, 36x on 900 mm, 48x on 1,200 mm and 52x on 1,300 mm.

On 1,500 mm it is 60x, and on 2,032 mm it is 81x.

One eyepiece, six telescopes, six answers.

Two rounding notes, so the table does not surprise you. Powers are rounded to whole numbers, so the 32 mm row reads 38x where the exact figure is 37.5x.

The site calculators keep the decimal. That applies under 100x.

The columns are not interchangeable. A 660 mm column belongs to a short refractor. A wide sky is the point there.

The 2,032 mm column belongs to a long Schmidt-Cassegrain. There a 25 mm already gives 81x.

Two eyepieces and two scopes make the point. A 10 mm on the 660 mm refractor is 66x.

The same 10 mm on the 2,032 mm is 203x.

The 25 mm pair is 26x and 81x. Nothing about the eyepiece changes between those cells. Only the tube length does.

The last line of the table is the ceiling. It is set by your aperture, not the eyepiece. The rule is 2 × aperture in mm.

That is 204x for a 102 mm refractor. 160x for an 80 mm one. 406x for 203 mm.

Five cells in the table sit past their own column's ceiling. A 4 mm on 900 mm reads 225x against a 160x limit.

A 4 mm on 1,300 mm reads 325x against 204x, and a 6 mm on the same tube reads 217x.

A 4 mm on 1,500 mm reads 375x against 300x. A 4 mm on 2,032 mm reads 508x against 406x.

Those five cells are what a spec sheet can print. The glass cannot deliver them.

They come from multiplying focal length by a Barlow factor.

No amount of glass fixes them.

Nothing in the table wastes the aperture either. The lowest useful power is aperture ÷ 7.

A dark-adapted pupil is about 7 mm across, so that is the number the floor comes from.

On 203 mm that is 203 ÷ 7 = 29x. On a 1,200 mm tube, a 29x power is a 41 mm eyepiece.

The slowest cell in the table is a 32 mm on 900 mm. That reads 28x, against a floor of 80 ÷ 7 = 11x.

Every row and column pair sits inside the useful band. The floor is never crossed.

If your telescope is not one of the six columns, the rule still holds. Divide its focal length by the eyepiece focal length.

Then check the answer against 2 × aperture.

A 2x Barlow doubles the effective focal length. The doubling happens before the eyepiece sees the light.

On a 1,200 mm tube that is 2,400 mm.

The 25 mm goes from 48x to 96x. The 10 mm from 120x to 240x. The 6 mm from 200x to 400x.

The field halves with the power. The 25 mm that showed 1.08° at 48x shows 0.54° inside the Barlow.

Nothing is gained in the sky, only in the eyepiece.

Table 2 repeats the ladder with the Barlow in the path. Its third column is the exit pupil, and it shows the cost.

25 ÷ 11.8 = 2.12 mm, about half of the 4.23 mm you get without it.

The focal ratio doubles with it, from f/5.9 to f/11.8.

The ceiling does not move, whatever you put in the eyepiece. A Barlow changes the focal length, not the aperture.

The glass is still 203 mm across.

So 406x is still 406x. That makes the 6 mm at 400x the most powerful combination you can reach on this scope.

It sits 6x of magnification under the limit.

The 4 mm at 600x is the row that fails. A 2x Barlow cannot lift a 4 mm past the ceiling of a 203 mm scope.

The extra magnification makes the image larger and softer, not clearer.

The 0.5 mm exit pupil floor also moves. Inside a 2x Barlow only eyepieces of 6 mm and longer stay above it.

6 ÷ 11.8 = 0.51 mm, while the 4 mm falls to 0.34 mm. That beam is too narrow for a dark-adapted eye.

Table 2 also shows which eyepieces survive the doubling. Nine of the ten rows stay inside 406x with a 2x Barlow on this scope.

Only the 4 mm at 600x fails.

The same arithmetic on another column changes the answer. A 2x Barlow on the 1,300 mm tube makes 2,600 mm of effective focal length.

A 10 mm there reaches 260x against a 204x ceiling. A 13 mm at 200x stays inside it.

The Barlow lens page has the mechanics and the 2x against 3x comparison.

Two numbers decide whether a power is usable. One is the exit pupil your eye receives.

The other is the true field you see.

Both come off the same reference scope. A 203 mm f/5.9 Dobsonian at 1,200 mm.

The exit pupil is the beam leaving the eyepiece, and it lands in your eye. It is eyepiece focal length divided by the focal ratio.

A 4 mm gives 0.68 mm. A 10 mm gives 1.69 mm. A 25 mm gives 4.23 mm, and a 32 mm gives 5.41 mm.

Write it the other way and you get the same number: 203 ÷ 48 = 4.23 mm.

Two limits matter here. Below 0.5 mm the beam is narrower than a dark-adapted pupil.

Extra magnification then only enlarges blur.

Above 7 mm the beam is wider than that pupil. Light is thrown away instead of collected.

The exit pupil shrinks as the power rises and the eyepiece gets shorter. That is the same relationship twice.

203 ÷ 300 = 0.68 mm, and 4 ÷ 5.91 = 0.68 mm. Pick whichever direction is easier for you to remember.

The true field is how much sky fits in the view. It is apparent field divided by power, and your eyepiece sets the apparent field.

We assume 52° for any eyepiece whose maker does not publish a figure. That is the standard Plössl number.

A 10 mm at 120x shows 52 ÷ 120 = 0.43°.

Where a maker does publish a figure, it beats the assumption. An eyepiece listed at 68° and used at 60x shows 68 ÷ 60 = 1.13°.

That is about a quarter of a degree wider than the 52° shortcut would give.

The 32 mm row is the exception inside our own assumptions. A 1.25 in barrel cuts the field at 27 mm.

27 ÷ 1,200 × 57.3 = 1.29°, narrower than the 1.39° the 52° shortcut gives at 37.5x. A 2 in barrel stops at 46 mm, which is 2.20° on this tube.

That limit only matters under about 24x.

For scale, the Moon is 0.52° across. At 25 mm the field is 1.08°. The whole disk fits with room around it.

At 10 mm the field is 0.43°, and the Moon runs past every edge.

The 0.52° Moon is the yardstick for every number in your column. A 0.43° field is smaller than the Moon. 1.08° is twice it, and 1.29° is two and a half.

The eyepiece calculator does both numbers for your own field stop.

Four to six millimetres is the high-power end of the ladder. On 1,200 mm a 4 mm is 300x with a 0.17° field.

A 6 mm is 200x with 0.26°. Exit pupils of 0.68 mm and 1.02 mm mean the light is spread thin.

These are the eyepieces for a small bright target, not a wide faint one.

The Celestron Omni 6 mm sits at the bottom of that band.

It gives 200x on a 1,200 mm tube, half of the 406x ceiling.

Eight to ten millimetres is the middle band. A 10 mm gives 120x with a 1.69 mm exit pupil and a 0.43° field.

An 8 mm gives 150x at 1.35 mm and 0.35°. Both sit above 1 mm, so the image keeps its brightness on a faint target.

The field stays narrow enough to work in.

Thirteen to twenty millimetres does two jobs. A 20 mm is 60x with a 0.87° field, and a 13 mm is 92x with 0.57°.

It is wide enough to hold a small group of stars. It is long enough to pull a planetary disk apart a little.

Twenty-five to thirty-two millimetres is the wide-field end. A 25 mm gives 48x and 1.08°, a 32 mm gives 38x and 1.29°.

At those powers the whole Moon fits. The 0.52° disk sits inside the field, and that is the reason to run them.

Three focal lengths cover most of a night on a 1,200 mm scope.

25 mm for the wide view, 10 mm for the middle, 6 mm for the high-power work.

On a 660 mm refractor they give 26x, 66x and 110x. On a 2,032 mm Schmidt-Cassegrain they give 81x, 203x and 339x.

Working the other way round starts from the power you need. Then you read back to a focal length.

That is what the eyepiece chooser is for.

The box says 675x. Work it backwards. That 675 is 900 mm of focal length.

A 3x Barlow and a 4 mm eyepiece do the rest.

The scope behind the number has a 60 mm (2.4 in) lens. Its ceiling is 2 × 60 = 120x.

So 675 ÷ 120 = 5.6. The printed power is more than five times past what your glass can use.

The same scope with a 25 mm eyepiece is 900 ÷ 25 = 36x. Now the gap is 675 ÷ 36 = 18.75.

A number on a box can be that far from the number you get.

Another label, another telescope. A 1,500 mm f/10 Schmidt-Cassegrain printing 450x would be 4,500 mm ÷ 10 mm.

So the 450 comes from a 3x Barlow on a 150 mm aperture. That scope's ceiling is 2 × 150 = 300x.

450 ÷ 300 = 1.5, so it is 1.5 times past the limit.

A 4 mm eyepiece on a 1,200 mm scope is 300x on its own. Add a 3x Barlow and the label would read 900x, against the ceiling your 203 mm glass sets.

Three steps get you a figure you can trust. Find your aperture in millimetres.

Find the focal length. If the box gives only a focal ratio, multiply the aperture by it.

Then divide that focal length by your eyepiece focal length. Compare the answer with 2 × aperture.

The rule works in either direction. Any printed power can be checked the same way.

The magnification calculator runs those steps on any pair of numbers you have.

It gives the exit pupil and the ceiling with the answer.

Every exit pupil and true field on this page comes off one scope. It is a 203 mm (8 in) at f/5.9, or 1,200 mm of focal length.

The 8-inch page has the brightness and resolution side of that same scope.

Four formulas do the work. Magnification = telescope focal length ÷ eyepiece focal length, with any Barlow factor on the numerator.

Exit pupil = eyepiece focal length ÷ focal ratio, the same as aperture ÷ magnification.

True field = apparent field ÷ magnification. Or field stop ÷ focal length × 57.3 when the barrel sets the limit.

The ceiling is 2 × aperture in mm, and the floor is aperture ÷ 7.

Apparent field is 52° whenever a maker does not publish one. That is the Plössl standard.

A published number beats the assumption. The X-Cel LX 25 mm is listed at 60°, so its 48x shows 1.25° of true field rather than 1.08°.

The six columns come from six spec sheets. 660 mm on a 102 mm f/6.5 refractor, and 900 mm on an 80 mm f/11 one.

1,200 mm on a 203 mm f/5.9 Dobsonian, and 1,300 mm on a 102 mm f/12.7 Maksutov.

1,500 mm on a 150 mm f/10 Schmidt-Cassegrain, and 2,032 mm on a 203 mm f/10 one.

All checked 2026-11-02.

Two rounding rules run through the tables. Powers are whole numbers, so a 32 mm on 1,200 mm reads 38x rather than 37.5x.

Exit pupils keep two decimals, so 32 ÷ 5.91 reads 5.41 mm.

Field stops are 27 mm for a 1.25 in barrel and 46 mm for a 2 in one. Those are the same limits the site calculators use.

The 0.5 mm to 7 mm exit pupil band comes from the dark-adapted pupil. The field stop limits come from the two barrel sizes.

Each column is a different kind of telescope, so one eyepiece behaves differently in each.

A 102 mm refractor at 660 mm is a wide-field instrument. The 2,032 mm Schmidt-Cassegrain at the far right is a long one.

The focal length is what changes between columns. The aperture is what sets the ceiling.