How magnification is calculated
Magnification equals the telescope’s focal length divided by the eyepiece’s focal length. A 1200 mm telescope with a 12 mm eyepiece gives 100x. Changing eyepieces changes magnification; the telescope itself does not have one fixed value. See focal length for more on the inputs. A Barlow lens multiplies the result, for example a 2x Barlow turns 100x into 200x.
Maximum useful magnification
A widely used rule of thumb is about 50x per inch of aperture, or roughly 2x per millimetre, under good conditions. So:
- 70 mm scope: about 140x
- 100 mm scope: about 200x
- 200 mm (8 inch) scope: about 400x
These are ceilings, not targets. Beyond them the image only gets larger and blurrier. The limit comes from diffraction, which sets how much detail a given aperture can show.
Minimum useful magnification
At very low power the beam leaving the eyepiece (the exit pupil) can be larger than your eye’s pupil, wasting light. Exit pupil equals aperture divided by magnification. If your pupil opens to about 7 mm or less, the lowest sensible magnification is around aperture in mm divided by 7, or a bit higher for older eyes. For a 100 mm scope that is roughly 14x.
Seeing and optical quality
Air turbulence (seeing) often limits magnification more than the telescope does. On many nights 150x to 250x is as high as is comfortable even in larger scopes. Poor optics, imperfect collimation, a shaky mount, or a tube not yet cooled to ambient temperature can also reduce the usable top end.
Is 200x or 40x good?
- 40x is a moderate power, useful for the Moon, large clusters and wide fields, and it works in almost any scope.
- 200x is a high power. It can work in a good scope of roughly 100 mm or larger on a steady night, for example on planets or double stars. In a small scope it will usually look dim and soft.
Binocular numbers: 40x60 vs 12x50
In binocular and spotting scope labels, the first number is magnification and the second is aperture in mm. Whether 40x60 beats 12x50 depends on use. The 40x60 magnifies more but has a narrower field and needs a steadier support, while 12x50 is easier to hold and gives a wider field. Neither is simply better.
Beware marketing claims
Boxes advertising “600x” on a small refractor are quoting an unrealistic mathematical maximum. Judge a telescope by its aperture and optical quality instead.
What magnification for which target
- Wide star fields and big clusters: low power, about 20x to 50x.
- Moon, smaller clusters, galaxies: medium, about 50x to 100x.
- Planets and double stars: higher, about 100x to 200x when steady.
Focal ratio influences which eyepieces you need; see fast vs slow telescopes.