\documentclass[a4paper,12pt]{article}
\usepackage[active,tightpage]{preview}
\begin{document}
\large

% image 1: wobbly subscript
\begin{preview}
  $ X^{1}_{2} X_{2} $
\end{preview}

% image 2: even subscript pushed down by null superscript
\begin{preview}
  $ X^{1}_{2} X^{}_{2} $
\end{preview}

% image 3: simple use of maths:
\begin{preview}
  \texttt{fi}\ensuremath\Rightarrow``fi''
\end{preview}
\end{document}

%%% Local Variables: 
%%% mode: latex
%%% TeX-master: t
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