Magnification vs. Aperture Explained Simply
Telescope boxes love big magnification numbers. Experienced observers ignore them. Here is why.
Aperture: the number that matters
Aperture is the diameter of the main lens or mirror. It decides:
- How much light is collected (faint objects need more light).
- How much fine detail can be resolved.
A 130mm mirror collects about 3.4 times more light than a 70mm lens.
Magnification: just the eyepiece
Magnification = telescope focal length ÷ eyepiece focal length. A 900mm telescope with a 10mm eyepiece gives 90x. Swap to a 25mm eyepiece: 36x. A 2x Barlow doubles either.
The limit
Beyond about 2× the aperture in mm, the image only gets bigger and dimmer, not more detailed. A "675x" claim on a 60mm telescope is marketing; its real limit is about 120x.
Exit pupil
Exit pupil = aperture ÷ magnification. Between about 0.7mm and 2mm suits planets; 3–6mm suits faint, large objects like nebulae. Above 7mm, light is wasted because it doesn't fit through your pupil.