How to vertically center the fraction line?

Question

I feel a bit weird because this is a really simple issue that apparently nobody has brought up so far but I find that very simple fractions produce output that looks vertically unbalanced. The following example uses Iwona as math font because there it's really obvious – with the gap between numerator and fraction line being almost 1.5 times as large as the space between denominator and the line.

\documentclass{standalone}
\usepackage[math]{iwona} % Iwona as math font
\begin{document}
$\frac{1}{2}\frac{1}{N}$
\end{document}

It's not completely a font issue though. I measured Computer Modern and the imbalance factor there is about 1.3, too.

I find this visually unpleasant and somewhat surprising – given how much LaTeX is hailed for its math typesetting, there is probably a reason for this imbalance. Either way, is there an easy way to fix this feature?

Answer 1

The default settings don't pack the numerator and denoimnator as close to the line as possible which gives a more consistent layout. In 1/2 the gap from the 1 to the bar is, as you say larger than the gap from the bar to the 2, but in 1/x the larger gap is below the bar, however 1/2 and 1/x are aligned consistently.

However you can change the font params to get a tighter spacing:

\documentclass{article}
\usepackage[math]{iwona} % Iwona as math font
\begin{document}
$a$
\typeout{\the\fontdimen9\textfont2}

$1+\frac{1}{2}+ \frac{1}{x}+\frac{1}{N}+\frac{1}{N^2} + \frac{1}{(a+b)}$


\fontdimen9\textfont2=2pt

\typeout{\the\fontdimen9\textfont2}
$1+\frac{1}{2}+ \frac{1}{x}+\frac{1}{N}+\frac{1}{N^2} + \frac{1}{(a+b)}$


\end{document}

See This answer for a list of font parameters.