Focal length
The distance over which the telescope brings light to focus. It affects image scale and eyepiece magnification.
Astronomy Tools
Free astronomy tool
Telescope & Eyepiece Calculator
Work out telescope focal length, focal ratio, magnification, exit pupil and approximate field of view before buying or combining equipment.
Telescope details
Enter the values printed on your telescope or specification sheet.
Diameter of the main mirror or lens
For example, enter 5 for f/5
Aperture × focal ratio
Focal length = aperture × focal ratio. With 130 mm at f/5, the focal length is 650 mm.
Barlow lens or focal reducer
Select 1× when no Barlow lens or reducer is being used.
Effective focal length
650 mm
Eyepieces
Add your eyepieces to compare their performance.
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Apparent field of view is usually printed as AFOV on the eyepiece specification. Leave it blank if unknown.
Understanding the results
The numbers help compare combinations, but observing conditions and optical quality still matter.
Magnification
Effective telescope focal length divided by eyepiece focal length. More power does not always mean more detail.
Exit pupil
The diameter of the light beam leaving the eyepiece. Large exit pupils give bright, wide views; very small ones become dim.
True field
An approximate width of sky visible through the eyepiece, calculated from apparent field divided by magnification.
Maximum useful magnification is an approximate upper limit of about 2× aperture in millimetres. Atmospheric seeing, collimation, optical quality and the object being viewed may impose a much lower practical limit.
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