Sizing of nested expressions in math mode

Question

Consider the following example in math mode:

\documentclass[a4paper,11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\begin{document}
\begin{equation}
  e^{i\frac{\theta_{\!j}}{2}}
\end{equation}
\end{document}

This is really looking ugly and this seems to be caused by j having approximately the same size as theta. How can I fix the size of j and why is this not automatically done?

Answer 1

There is a \scriptstyle, and a \scriptscriptstyle, but no \scriptscriptscriptstyle. Presumably this is to prevent the type from becoming so small as to be unreadable. I would write it as follows, so that it has two levels of sub/superscripting, but not three.

\documentclass[a4paper,11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\begin{document}
\begin{equation}
  \mathrm{e}^{\mathrm{i}\theta_j/2}
\end{equation}
\end{document}

Answer 2

There are many ways, for example:

\documentclass[a4paper,11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{mleftright}
\newcommand*{\img}{\mathrm{i}}
\newcommand*{\euler}{\mathrm{e}}
\begin{document}
\begin{gather}
  e^{i\frac{\theta_{\!j}}{2}}
  = e^{i\,\theta_{\!j}/2}
  = \exp(i\frac{\theta_{\!j}}{2})
  = \exp\Bigl(i\frac{\theta_{\!j}}{2}\Bigr)
  = \exp\mleft(i\frac{\theta_{\!j}}{2}\mright)
  = \exp(i\,\theta_{\!j}/2)
\\
  \euler^{\img\frac{\theta_{\!j}}{2}}
  = \euler^{\img\,\theta_{\!j}/2}
  = \exp(\img\frac{\theta_{\!j}}{2})
  = \exp\Bigl(\img\frac{\theta_{\!j}}{2}\Bigr)
  = \exp\mleft(\img\frac{\theta_{\!j}}{2}\mright)
  = \exp(\img\,\theta_{\!j}/2)
\end{gather}
\end{document}