🔭 Telescope Magnification Calculator

Enter your telescope's focal length and an eyepiece's focal length to calculate the magnification that eyepiece provides.

Telescope Focal Length (mm)

Eyepiece Focal Length (mm)

How the magnification is worked out

Only the two focal lengths matter, both in millimetres:

magnification = telescope focal length ÷ eyepiece focal length

The page opens on 1000 mm and 25 mm, which is 40×. Put a 6 mm eyepiece in the same tube and 1000 ÷ 6 gives 166.7×. Aperture is not part of the sum, so a 60 mm department-store refractor of 1000 mm focal length reports the same 40× as a 200 mm reflector of the same focal length — it simply shows far less at that power. Answers are rounded to one decimal, and a focal length of zero or less is refused.

Picking an eyepiece

On a 1000 mm scope the common sizes land here:

Eyepiece	Magnification
32 mm	31.3×
25 mm	40.0×
10 mm	100.0×
6 mm	166.7×

Two ceilings sit outside this calculator and both need the aperture:

Highest useful magnification is about 2× the aperture in millimetres — roughly 400× on a 200 mm scope. Steady air is rarer than good optics, so most nights stop you nearer 200–250×.
Exit pupil is aperture ÷ magnification. That 200 mm scope at 40× gives a 5 mm beam, bright and easy on the eye; at 400× it is 0.5 mm, and the view turns dim and grainy.

A Barlow is not in the inputs. Work out the plain figure first, then multiply by the Barlow’s factor — a 2× Barlow turns that 10 mm eyepiece into 200×, the same as a 5 mm.

Frequently asked questions

What magnification does a 25 mm eyepiece give?

It depends entirely on the telescope. In a 1000 mm scope it is 40×, in a 650 mm short refractor 26×, and in a 2800 mm Schmidt-Cassegrain 112×.

Why does the image get worse at high magnification?

You are magnifying the atmosphere as well as the target, and the fixed amount of light the aperture collects is spread over a bigger image. Past roughly 2× the aperture in millimetres nothing new appears, just a larger, dimmer, softer version of what you already had.

Do I need the shortest eyepiece I can buy?

No. Most deep-sky objects look best at 30–80×, where the field is wide and the exit pupil generous. High power is for the Moon, planets and splitting double stars, and only when the seeing is steady.