Magnification is the telescope's focal length divided by the eyepiece's focal length, using the same units. A 900 mm telescope with a 25 mm eyepiece gives 36 times magnification. The same telescope with a 10 mm eyepiece gives 90 times.
Start at low power
A low-power view usually makes a target easier to find and keep in view. Once it is centred and focused, try a shorter-focal-length eyepiece if the target is bright enough and the image remains steady. Recheck focus after changing eyepieces.
The largest number is not automatically the best view. More power spreads an extended object's light over a larger apparent area and makes vibration and atmospheric movement more obvious. Large clusters can be more satisfying in a broad field than at high magnification, where much of the object no longer fits.
Change one thing at a time
If planetary detail disappears as you increase power, go back a step and wait for steadier moments. If every view shakes when you touch the focus control, improve the support or allow the vibration to settle. Do not assume a new eyepiece can repair poor atmospheric conditions.
Record the telescope, eyepiece and resulting power beside a sketch or observation. That makes a comparison next month useful. Follow the instrument maker's limits and operating instructions rather than treating an advertised maximum as a target. Try equipment at a club event when possible before choosing accessories for a specific observing task.
Sources and review
Reviewed October 22, 2026 by PiSpace Sky editorial. Date-sensitive details can change.