### 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 if there is a secondary lens or mirror, where the
focal plane would be if the secondary were removed).
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/R_{1}+1/R_{2})

*f* - focal length
- n -
**refractive index** of the lens material (a fractional measure showing how much it affects light's speed).
- R
_{1}, R_{2} - the radii of curvature of the two sides of the lens.

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

1/*f* = (n-1)(1/R_{1}+1/R_{2}+(n-1)d/(nR_{1}R_{2}))

- d - thickness of the lens, through the center, specifically where the lens surfaces are perpendicular to a line straight through.

(*telescopes,optics*)
**Further reading:**

http://en.wikipedia.org/wiki/Focal_length

http://en.wikipedia.org/wiki/F-number

**Referenced by pages:**

Cassegrain reflector

field curvature

focal plane

Herschelian telescope

Lynx

NuSTAR

plate scale

Schiefspiegler

Schmidt camera

Schmidt-Newton telescope (SNT)

SOFIA

spherical aberration

WFIRST

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