\latexonly{$\alpha$}\latexignore{\emph{alpha}} and the \emph{focal
length}. It is calculated as \latexonly{$4\,f \tan (\alpha / 2)$}
\latexignore{\emph{4 x F x tan(\alpha/2)}}
\latexonly{$\alpha$}\latexignore{\emph{alpha}} and the \emph{focal
length}. It is calculated as \latexonly{$4\,f \tan (\alpha / 2)$}
\latexignore{\emph{4 x F x tan(\alpha/2)}}