Telescope Aperture and Magnification Chart
A telescope's aperture, the diameter of its main lens or mirror, sets a hard ceiling on useful magnification regardless of what eyepiece you attach, and boxes advertising 300x or 600x power on a small aperture are marketing a number the optics can never deliver cleanly. This chart lists the practical maximum magnification for common apertures using the standard 50x-per-inch (about 2x per millimeter) guideline, plus which eyepiece focal lengths reach useful magnifications on each.
The maximum useful magnification formula
The widely used rule of thumb is roughly 50x magnification per inch of aperture, or about 2x per millimeter, under good atmospheric conditions; pushing past this floods the image with dim, blurry, unresolved light rather than showing more detail. This ceiling exists because aperture determines how much light the telescope gathers and how finely it can resolve detail, and no eyepiece can extract resolution the optics never captured in the first place.
Real-world viewing conditions usually cut the practical ceiling well below the theoretical maximum: atmospheric turbulence limits most locations to a genuinely useful 200-300x regardless of aperture on an average night, and only exceptionally steady air supports magnifications above that even with a large telescope.
Maximum useful magnification by common aperture
70mm (2.75 inch) aperture: roughly 140x maximum useful magnification. A common beginner refractor size, better suited to wide-field views of the Moon and bright planets than high-power detail work.
90mm (3.5 inch) aperture: roughly 180x maximum useful magnification. A step up that starts to show genuine planetary detail like Saturn's rings and Jupiter's cloud bands under good conditions.
114mm (4.5 inch) aperture: roughly 228x maximum useful magnification. A common beginner reflector size offering a meaningful jump in light-gathering over small refractors at a similar price point.
150mm (6 inch) aperture: roughly 300x maximum useful magnification. Often considered the sweet spot for a serious beginner or intermediate telescope, balancing genuine performance with manageable size and weight.
200mm (8 inch) aperture: roughly 400x maximum useful magnification, though atmospheric conditions usually cap real-world use around 250-300x on most nights. A popular size for dedicated hobbyists wanting real deep-sky and planetary performance.
250mm (10 inch) and larger: theoretical maximums exceed 500x, but atmospheric turbulence, not the optics, becomes the limiting factor on all but the steadiest nights, so the practical gain over an 8 inch telescope shows up mainly in light-gathering for faint deep-sky objects rather than in usable magnification.
Choosing eyepiece focal length for a target magnification
Magnification equals the telescope's focal length divided by the eyepiece's focal length, so a telescope with a 1000mm focal length paired with a 10mm eyepiece yields 100x magnification, and swapping to a 25mm eyepiece drops that to 40x. Check your telescope's specific focal length, printed on the tube or in its manual, before assuming any eyepiece's stated focal length maps to a particular magnification, since the same eyepiece produces very different magnifications on telescopes with different focal lengths.
A useful starter eyepiece set covers low power (25-32mm, wide field views for finding objects and viewing large deep-sky targets), medium power (10-15mm, general planetary and lunar viewing), and high power (5-7mm, used only on the steadiest nights near your telescope's practical ceiling), which covers the realistic range most aperture sizes above can actually use well.