By Zencalcy Editorial TeamLast reviewed About us
A telescope's magnification is the objective focal length divided by the eyepiece focal length. Short eyepieces magnify more, but they also narrow the field and dim the image, so useful magnification is limited by aperture and seeing.
Angular magnification from objective and eyepiece focal lengths.
M = f_obj / f_eye
M is angular magnification, f_obj the objective focal length and f_eye the eyepiece focal length. The image is inverted in a simple refractor.
| Symbol | Meaning |
|---|---|
| M | Angular magnification, dimensionless. |
| f_obj | Focal length of the objective. |
| f_eye | Focal length of the eyepiece. |
M = 1000 / 25 = 40×. A 10 mm eyepiece would give 100×, but the image would be dimmer and harder to hold steady.
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No. Beyond about 50× per inch of aperture, the image only gets dimmer and blurrier. Useful magnification is limited by aperture and atmospheric seeing.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Content updated: September 2026