Telescope Magnification Calculator
Estimate the magnification of a simple telescope-and-eyepiece combination using focal lengths.
How to use this calculator
- Enter objective focal length (mm), eyepiece focal length (mm); check the units and use values from the same period or scenario.
- Review the result and intermediate values; change one input at a time when comparing scenarios.
- Keep the stated formula and assumptions with any result you share.
Formula and method
Magnification = objective focal length ÷ eyepiece focal length.
- Nominal magnification is 60×.
- Useful magnification is limited by aperture, atmosphere, optical quality, target contrast and mount stability.
Worked examples
Worked example
A 900 mm telescope objective is paired with a 15 mm eyepiece.
Nominal magnification is 60×.
Nominal magnification is 60×.
Understanding the result
Nominal magnification is 60×.
Useful magnification is limited by aperture, atmosphere, optical quality, target contrast and mount stability.
Important notes
Useful magnification is limited by aperture, atmosphere, optical quality, target contrast and mount stability.
Use measured inputs and verify that the model matches the situation before making an operational, financial or technical decision.
Frequently asked questions
Does higher magnification always show more detail?
No. Beyond the useful range, the view may become dim or blurry.
Does this include a Barlow lens?
Not directly; use the effective focal length for the optical configuration.
What should I check before relying on this result?
Verify units, input definitions and assumptions against your real data or the relevant product, service or professional guidance.
Related calculators
- Camera Crop Factor Equivalent Focal Length Calculator — Convert a lens’s actual focal length into a 35 mm equivalent using the camera crop factor.
- Thin Lens Focal Length Calculator — Calculate focal length and optical power from positive object and real-image distances using the thin-lens relation.
- Optical Magnification Calculator — Calculate linear magnification from image height and object height.
- Telescope Exit Pupil Calculator — Estimate the exit-pupil diameter from telescope aperture and magnification.
- Lens Depth of Field Calculator — Estimate near and far focus limits from focal length, aperture, subject distance and an assumed circle of confusion.
- API Error Rate Calculator — Calculate the fraction of API requests that meet your defined error criterion in a measured period.
Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
- How to Calculate a Z-Score — Learn how a z-score shows how far a value is from the mean in standard-deviation units.
- How to Calculate Salary from an Hourly Rate — Convert hourly pay into weekly, monthly, and annual estimates and understand the assumptions behind the result.
- Percent Difference — Understand percent difference, when it is appropriate, and how it differs from percentage change and percent error.
This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.