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F- Fa Fe Fi Fo Fr Fw
Fie Fir Fis

Field of View

(usually horizontal) extend of an object that’s visible on a sensor.

Sometimes the HFOV (horizontal field of view), sometimes the VFOV (Vertical Field of View) and sometimes the DFOV (Diagonal Field of View) matters. It’s of utmost importance to state clearly which matters.

Fisheye Types

Various types of fisheye lenses are available.
Here a short overview:

Fisheye Types:

Type gnomonic stereographic F-Theta equal area orthographic
lens class wide angle Fisheye Fisheye Fisheye Fisheye
mapping function r(\theta) = f \cdot tan \theta r(\theta) = 2 \cdot f \cdot tan \frac{\theta}{2} r(\theta)=f \theta r(\theta) = 2 \cdot f \cdot sin \frac{\theta}{2} r(\theta) = f \cdot sin \theta
normalized mapping function h(\theta)=\frac{r(\theta)}{f} h(\theta) = tan \theta h(\theta) = 2 \cdot tan \frac{\theta}{2} h(\theta)=\theta h(\theta) = 2 \cdot sin \frac{\theta}{2} h(\theta) = sin \theta
meridional scaling S_{m}={\frac {\mathrm {d} \mathbf {h} (\theta )}{\mathrm {d} \theta }} S_{m}={\frac {1}{\cos ^{2}\theta }} S_{m}={\frac {1}{\cos ^{2}{\frac {\theta }{2}}}} S_{m}=1 S_{m}=\cos {\frac {\theta }{2}} S_{m}=\cos \theta
sagittal scaling S_{s}={\frac {\mathbf {h} (\theta )}{\sin \theta }} S_{s}={\frac {1}{\cos \theta }} S_{s}={\frac {1}{\cos ^{2}{\frac {\theta }{2}}}} S_{s}={\frac {\theta }{\sin \theta }} S_{s}={\frac {1}{\cos {\frac {\theta }{2}}}} S_{s}=1
effective scaling S=\sqrt{S_{m} \cdot S_{s}} S={\frac {1}{\sqrt {\cos ^{3}\theta }}} S={\frac 1{\cos ^{2}{\frac \theta 2}}} S={\sqrt {\frac {\theta }{\sin \theta }}} S=1 S={\sqrt {\cos \theta }}
N from S_{m}=S_{s}^{N} 2 1 0 -1 \infty
Balance Deform. vs. Scaling B={\frac {2\,(N-1)}{N+1}} \frac{2}{3} 0 -2 \infty 2
Curvature C={\frac {N-2}{3-N}} 0 -{\tfrac 12} -{\tfrac 23} -{\tfrac 34} -1
Deformation D={\frac {S_{m}}{S_{s}}} D={\frac 1{\cos \theta }} D=1 D={\frac {\sin \theta }\theta } D=\cos ^{2}{\frac \theta 2} D=\cos \theta
maintains – angles angular distances areas planar illuminance
AOV \alpha_{max} <180° <360° >= 360° 360° 180°
\alpha_{weak} 54° 94° 115° 94° 66°
\alpha_{medium} 75° 131° 159° 131° 90°
\alpha_{strong} 102° 180° 217° 180° 120°

Source of the various angles and formulae: wikipedia

Here the angle  \theta is measured in Radians !

equal area lenses are also called “equisolid angle” or “flächentreu”
F-Theta lenses are also called “equidistant“, “linear scaled“, “äquidistant” or “angular”
sterographic lenses are also called “ conform“or “ winkeltreu”
orthographic lenses are also called” hemispherical“or “orthographisch“

For details see:
equal area
F-Theta
gnomonic
orthographic
stereographic