Telescope Magnification Calculator

Get magnification, exit pupil, true field of view, and f-ratio for any scope and eyepiece combination.

How it works

Enter your scope's focal length and aperture (both printed on the tube or in the manual, in millimeters) plus the focal length and apparent field of view of whichever eyepiece you're considering. Magnification is just scope focal length divided by eyepiece focal length: a longer eyepiece number gives lower power, a shorter one gives higher power. Everything else the calculator shows follows from that one number.

Worked example: a common beginner scope is a 130 mm Newtonian with a 900 mm focal length, paired with a 25 mm Plössl (a 50° apparent field is typical for that eyepiece style). That gives 900 / 25 = 36x magnification. The exit pupil, the width of the beam of light hitting your eye, comes out to 130 / 36 = 3.6 mm, comfortably inside the range a dark-adapted eye can use. True field of view is 50° / 36 = 1.4°, close to three full Moon diameters. The scope itself is f/6.9 (900 / 130), a middle-of-the-road ratio that works fine for both planets and wider deep-sky views. Swap in a shorter eyepiece and every number shifts: magnification climbs, the exit pupil shrinks, and the true field narrows.

The max useful magnification line uses the classic rule of about 2x per millimeter of aperture, so a 130 mm scope tops out around 260x on paper. In practice, steady air rarely supports more than roughly 250x no matter how big your aperture is, and pushing past the max useful figure just spreads the same light over a bigger, dimmer, softer image.

FAQ

What if my magnification is above the max useful number?

The image will look dim, soft, and harder to focus rather than more detailed. This usually means the eyepiece focal length is too short for the aperture, or the atmosphere that night just won't support high power. Step up to a longer focal length eyepiece and the view will usually look better, not worse.

Why does exit pupil matter?

A young, fully dark-adapted eye can open to around 7 mm, so an exit pupil near that size delivers the brightest, widest view for faint deep-sky objects. Once the exit pupil drops much below about 0.5 mm the image goes dim and floaters in your eye become distracting, which is a practical ceiling on usable magnification even before you hit the 2x-per-mm rule.

Do I need to know my eyepiece's apparent field of view?

It's usually printed in the eyepiece specs or the box; 50° is a safe default for a basic Plössl if you can't find it. True field of view (how much actual sky you see) scales directly with it, so a wide-angle 68° or 82° eyepiece at the same magnification shows noticeably more sky than a 50° one.

Does a bigger aperture always mean more magnification?

Aperture raises the ceiling on max useful magnification and gathers more light, but the magnification you actually get at any moment still depends on which eyepiece is in the focuser. A bigger scope with a long eyepiece can still give a low-power, wide view; it just also has the option to go higher when the eyepiece and the sky both allow it.

For more on how aperture and eyepieces interact, see aperture vs. magnification, how eyepieces work, and our beginner telescope buying guide.