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Focal Length

(length from optical element to focal plane)

A telescope's Focal Length is the distance from the primary lens or mirror to the Focal Plane (or where the focal plane would be, in the case where there is also a secondary lens or mirror). The reciprocal of the focal length is called the optical Power.

The Focal Ratio (aka F-ratio, or for cameras, the F-stop or F-number) is the ratio of the focal length with the diameter of the Aperture. For cameras, the focal ratio is important for determining the depth of field. For telescopes, it is important for determining the Field of View: other elements being the same, the smaller the focal ratio, the larger the FOV, and telescopes aiming at Surveys often are given short focal ratios (e.g., Schmidt Cameras).

The Lensmaker's Formula (or Lensmaker's Equation) allows calculation of the focal length of a lens;

1/f ≈ (n-1)(1/R1+1/R2)
  • f - focal length
  • n - Refractive Index of the lens material (a fractional measure showing how much it affects light's speed).
  • R1, R2 - the radii of curvature of the two sides of he mirror.

A more accurate version, significant if the lens is not thin:

1/f = (n-1)(1/R1+1/R2+(n-1)d/(nR1R2))
  • d - thickness of the lens, through the center, specifically where the lens surfaces are perpendicular to a line straight through.


Referenced by:
Cassegrain Reflector
Focal Plane
Schmidt Camera
Schmidt-Newton Telescope