For parallel geometry scanning, the focal length doesn't matter.
However, divergent geometry scanning (equilinear and equiangular),
the \emph{focal length ratio} should be set at \texttt{2} or more
For parallel geometry scanning, the focal length doesn't matter.
However, divergent geometry scanning (equilinear and equiangular),
the \emph{focal length ratio} should be set at \texttt{2} or more