Telescope Magnification Calculator

Aperture and focal length are printed on the tube or in the manual. A typical line reads D=203mm F=1200mm.

If the label gives only the focal ratio, multiply the aperture by that number. 203 times 5.9 is about 1200 mm.

Magnification is one division: telescope focal length over eyepiece focal length. Multiply by the Barlow after that. No Barlow means times 1.

Swap to 10 mm and you get 120x, with a 1.7 mm exit pupil.

A 6 mm eyepiece gives 200x and a 1.0 mm pupil. A 15 mm eyepiece gives 80x and a 2.5 mm pupil.

To check the maximum magnification a box prints, double the aperture in millimeters. Focal length stays out of that cap.

Sky & Telescope states 50 times the inches, or 2 times the millimeters. Twice 203 mm is 406x.

The inch rule gives 400x for 8 in. The millimeter rule gives 406x. Treat both as the same ceiling.

On 1200 mm, a 6 mm eyepiece plus a 2x Barlow lands at 400x, next to that ceiling.

A dark-adapted eye opens to about 7 mm (Sky & Telescope), and Tele Vue tells reflector owners to stay under a 7 mm exit pupil.

Your pupil cannot take a wider beam than that. Divide 203 by 7 and you get 29x.

That floor wants about a 41 mm eyepiece. In a 1.25 in barrel the 27 mm stop squeezes it to about 38° apparent field, so that focal length is usually bought in 2 in.

A 32 mm 1.25 in eyepiece gives you 37.5x (about 38x) and a 5.4 mm pupil.

A 5 mm pupil, the adult example in the pupil primer, lifts your floor to 40.6x.

With that 5 mm pupil, the 32 mm (37.5x, 5.4 mm exit pupil) wastes part of the aperture. You are using about 188 mm, which is 37.5 times 5.

Exit pupil equals aperture divided by power. It also equals eyepiece focal length divided by the focal ratio.

On a reflector, stay between 0.5 mm and 7 mm. Tele Vue sets both edges. Under 0.5 mm you only enlarge blur.

At 406x the pupil is 0.50 mm. Save that power for the steadiest nights.

True field equals apparent field divided by power. With no field stop on the sheet, we use 52°.

At 48x, 52 divided by 48 is 1.08°. The Moon averages about 0.52° across (NASA's fact sheet gives 1,896 arcsec).

Tele Vue's stricter formula is field stop over focal length, times 57.3. Use it when the stop is printed.

A 1.25 in barrel caps the field stop at 27 mm. Tele Vue's 32 mm Plössl uses that stop, so you will not get a wider 1.25 in field.

On 1200 mm, 27 divided by 1200, times 57.3, is 1.29°. The 52° shortcut said 1.39°. Use the stop.

Our assumptions

• Aperture 203 mm, from 8 times 25.4, rounded.
• Focal length 1200 mm at f/5.9, called 8 in f/6.
• Barlow starts at 1x. You can set 2x or 3x.
• Use 52° only when the field stop is missing.
• Max is 2 times aperture in mm. Min is aperture over 7.
• The Moon comparison uses 0.52°, the same value as our calculators.
• Seeing is not an input. See the 30x-per-inch note below.
• The 675x example uses a 60 mm, 900 mm tube, f/15.
• Formula pages were checked on 2026-10-17.

When 300x fits

Whether 300x works depends on the eyepiece and the aperture. On 1200 mm, a 4 mm eyepiece gives 300x.

The pupil is 0.7 mm, and 300 sits under the 406x cap. You are inside the rule on 203 mm.

Tele Vue notes the air usually caps planetary power near 300x, whatever the aperture. On a soft night, stop closer to 240x.

On a 70 mm, 700 mm refractor the cap is 140x. You are 2.1 times past that cap at 300x.

The pupil there is 0.23 mm, under the 0.5 mm edge. The image is dim and soft; you are enlarging diffraction blur.

The box says 675x. That's 900 mm × 3 (a 3x Barlow) ÷ 4 mm. On a 60 mm (2.4 in) lens the ceiling is about 120x. At 675x the exit pupil is 0.09 mm. 675x is not usable on that lens: it is more than five times the 120x ceiling.

The same 25 mm eyepiece is 48x on 1200 mm and 60x on 1500 mm. Focal length does that.

A 6 in mirror is 152 mm here, from 6 times 25.4 rounded, and 1200 mm makes it f/7.9.

The 102 mm refractor is 663 mm, f/6.5. The 127 mm Mak is 1500 mm, f/11.8.

The 6 in row shares the 8 in powers, because both focal lengths are 1200 mm. Its pupil at 48x is 3.2 mm, and its cap is 304x.

The 102 mm tube stays under a 204x cap at 6 mm (110.5x). The Mak at 6 mm is 250x, against a 254x cap. Add 2x and you jump to 500x.

A bright suburb and a dark site

The formulas do not change with the sky. The row you should use does change.

Bortle class 6 is a bright suburban sky, and class 7 is suburban-urban. Class 3 is rural. Class 4 is the rural-suburban transition.

John Bortle set those classes in Sky & Telescope. We use 6 to 7 for a bright yard and 3 to 4 for a dark site.

On a class 6 night, a 5.4 mm pupil shows a bright background. Step to 15 mm. You get 80x and a 2.5 mm pupil on 1200 mm.

The smaller pupil dims the background. We treat 80x to 120x as the working range from a class 6 yard, well under the 406x math cap.

Ordinary air is stricter. A Sky & Telescope formulas note uses about 30 times per inch as a practical cap. On 8 in that is 240x.

On 1200 mm, 240x is a 5 mm eyepiece and a 0.8 mm pupil.

At a class 3 or 4 site, use the 32 mm if your pupil can take the 5.4 mm beam. You get 37.5x (about 38x) and 1.29° from the 27 mm stop.

Planets follow the air, not the Bortle class. Find and center the planet at about 50x, a 24 mm eyepiece on 1200 mm, then step up.

Tele Vue's eyepiece guide puts planetary and double-star work in at least the 150x range. On 1200 mm that is an 8 mm eyepiece and a 1.4 mm pupil.

An 8 to 24 mm zoom covers that span. At 24 mm you get 50x and a 4.1 mm pupil.

At 8 mm on this 1200 mm tube you get 150x and a 1.4 mm pupil.

Three slips that wreck the number

Slip one is the printed power. Divide before you trust it. The 675x label on 60 mm failed above.

Slip two is one eyepiece for every target. On 1200 mm, keep 25 mm at 48x, 10 mm at 120x, and 6 mm at 200x when the air holds.

Slip three is the barrel. A 2 in barrel allows a field stop up to 46 mm. A 1.25 in barrel caps the field stop at 27 mm. Tele Vue lists both limits.

A 2x on the 25 mm lands at 96x, the same power as a 12.5 mm eyepiece, so those two steps repeat. A 3x on the 6 mm is 600x. Stop there.

The Barlow lens page runs 2x and 3x on these focal lengths. Focal-length sets sit on how to choose telescope eyepieces. Zoom eyepieces and full kits are compared on telescope eyepiece kits compared.

For true field with your own eyepiece's apparent field, use the eyepiece calculator.

Past 406x you only magnify diffraction blur, even in perfect air. You do not see more detail. Sky & Telescope explains that limit in the pupil primer.