Why Telescope Magnification Claims Are Misleading
A telescope's useful magnification ceiling is roughly 2x its aperture in millimeters (50x per inch) — a 70mm telescope tops out near 140x regardless of what "600x" is printed on the box. Magnification past that point just makes the image bigger, dimmer, and blurrier; it adds no new detail.
Magnification is simple arithmetic: divide the telescope's focal length by the eyepiece's focal length. Any telescope can be pushed to a huge magnification number just by swapping in a very short-focal-length eyepiece or adding a Barlow lens. But the telescope's aperture — the diameter of its main lens or mirror — caps how much light it gathers and how finely it can actually resolve detail. Once you push magnification past roughly 2x the aperture in millimeters, you're not seeing more detail; you're seeing the same limited detail stretched across a bigger, dimmer, blurrier image.
That's why a "70mm, 600x" telescope box is describing a fantasy: that scope's aperture caps useful magnification near 140x. The 600x figure is technically achievable with the right eyepiece stack, but the resulting image would be unusably dim and soft. The number that actually predicts what you'll see is aperture, not magnification — see the full formula table in The Science Gear Truth Numbers.
Frequently Asked Questions
Roughly 2x the aperture in millimeters (about 50x per inch). A 70mm telescope has a practical ceiling near 140x; push past that with a strong eyepiece or Barlow and the image only gets bigger, dimmer, and blurrier.
Because it's an easy-to-understand-sounding number that sells on a store shelf, even though it's calculated from an unrealistic eyepiece/Barlow combination the aperture can't actually resolve well.
Aperture (in millimeters or inches) and mount stability. Aperture sets the real ceiling on both brightness and resolvable detail; a shaky mount will ruin the view of even a great telescope.