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Telescope Magnification Calculator.

Calculate angular magnification from objective and eyepiece focal lengths.

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Set your values

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Objective focal length

Eyepiece focal length

Magnification

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Results update automatically as you type.

Use Cases

Choosing the right eyepiece

Determine the magnification you'll get with different eyepieces on your telescope to match your observing goals, such as planetary detail or wide-field views.

Example: With a 1200 mm objective, a 25 mm eyepiece gives 48x, while a 10 mm gives 120x.

Checking magnification limits

Verify that your chosen eyepiece doesn't exceed the telescope's useful magnification limit (usually about 2x the aperture in mm) to avoid blurry images.

Example: For a 200 mm aperture, keep magnification under 400x; a 5 mm eyepiece on a 2000 mm scope gives 400x.

Frequently Asked Questions

How is telescope magnification calculated?: Magnification is calculated by dividing the objective focal length (in mm) by the eyepiece focal length (in mm). For example, a 1000 mm objective with a 10 mm eyepiece gives 100x magnification.
What units should I use for the focal lengths?: Both focal lengths must be in millimeters (mm). The calculator uses the same unit for both, so the result is a dimensionless magnification factor.
Can I use this calculator for any telescope?: Yes, as long as you know the focal lengths of the objective (or primary mirror) and the eyepiece. It works for refractors, reflectors, and catadioptric telescopes.

Tips & Common Mistakes

Tips

Always use the same unit (mm) for both focal lengths to get an accurate magnification ratio.
For the sharpest views, start with a low magnification eyepiece (longer focal length) and increase as needed.
Remember that magnification is not the only factor; aperture and atmospheric conditions also affect image quality.
Use this calculator to compare eyepieces before buying to ensure they suit your telescope's focal length.

Common Mistakes to Avoid

Mixing units, such as using meters for objective and millimeters for eyepiece, which leads to incorrect results.
Assuming higher magnification always means better views; too high magnification can result in dim, blurry images.
Forgetting that the calculator gives angular magnification, not the telescope's resolving power or field of view.

Last updated: August 13, 2026

Use Cases

Choosing the right eyepiece

Determine the magnification you'll get with different eyepieces on your telescope to match your observing goals, such as planetary detail or wide-field views.

Example: With a 1200 mm objective, a 25 mm eyepiece gives 48x, while a 10 mm gives 120x.

Checking magnification limits

Verify that your chosen eyepiece doesn't exceed the telescope's useful magnification limit (usually about 2x the aperture in mm) to avoid blurry images.

Example: For a 200 mm aperture, keep magnification under 400x; a 5 mm eyepiece on a 2000 mm scope gives 400x.

Frequently Asked Questions

How is telescope magnification calculated?

Magnification is calculated by dividing the objective focal length (in mm) by the eyepiece focal length (in mm). For example, a 1000 mm objective with a 10 mm eyepiece gives 100x magnification.

What units should I use for the focal lengths?

Both focal lengths must be in millimeters (mm). The calculator uses the same unit for both, so the result is a dimensionless magnification factor.

Can I use this calculator for any telescope?

Yes, as long as you know the focal lengths of the objective (or primary mirror) and the eyepiece. It works for refractors, reflectors, and catadioptric telescopes.

Tips & Common Mistakes

Tips

Always use the same unit (mm) for both focal lengths to get an accurate magnification ratio.
For the sharpest views, start with a low magnification eyepiece (longer focal length) and increase as needed.
Remember that magnification is not the only factor; aperture and atmospheric conditions also affect image quality.
Use this calculator to compare eyepieces before buying to ensure they suit your telescope's focal length.

Common Mistakes to Avoid

Mixing units, such as using meters for objective and millimeters for eyepiece, which leads to incorrect results.
Assuming higher magnification always means better views; too high magnification can result in dim, blurry images.
Forgetting that the calculator gives angular magnification, not the telescope's resolving power or field of view.

Use Cases

Choosing the right eyepiece

Determine the magnification you'll get with different eyepieces on your telescope to match your observing goals, such as planetary detail or wide-field views.

Example: With a 1200 mm objective, a 25 mm eyepiece gives 48x, while a 10 mm gives 120x.

Checking magnification limits

Verify that your chosen eyepiece doesn't exceed the telescope's useful magnification limit (usually about 2x the aperture in mm) to avoid blurry images.

Example: For a 200 mm aperture, keep magnification under 400x; a 5 mm eyepiece on a 2000 mm scope gives 400x.

Frequently Asked Questions

How is telescope magnification calculated?

Magnification is calculated by dividing the objective focal length (in mm) by the eyepiece focal length (in mm). For example, a 1000 mm objective with a 10 mm eyepiece gives 100x magnification.

What units should I use for the focal lengths?

Both focal lengths must be in millimeters (mm). The calculator uses the same unit for both, so the result is a dimensionless magnification factor.

Can I use this calculator for any telescope?

Yes, as long as you know the focal lengths of the objective (or primary mirror) and the eyepiece. It works for refractors, reflectors, and catadioptric telescopes.

Tips & Common Mistakes

Tips

Always use the same unit (mm) for both focal lengths to get an accurate magnification ratio.
For the sharpest views, start with a low magnification eyepiece (longer focal length) and increase as needed.
Remember that magnification is not the only factor; aperture and atmospheric conditions also affect image quality.
Use this calculator to compare eyepieces before buying to ensure they suit your telescope's focal length.

Common Mistakes to Avoid

Mixing units, such as using meters for objective and millimeters for eyepiece, which leads to incorrect results.
Assuming higher magnification always means better views; too high magnification can result in dim, blurry images.
Forgetting that the calculator gives angular magnification, not the telescope's resolving power or field of view.