You should Avoid using \eqnarray
, and rather use equation environments supplied by amsmath
. Here's one example that puts \iff
flush with the left margin (using flalign*
), while centering the rest of the equation:

\documentclass{article}
\usepackage{showframe}% http://ctan.org/pkg/showframe
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{flalign*}
&& e^z &= e^w & \\
\iff && log(e^z) &= log(e^w) \\
\iff && ln|e^z| + i\cdot Arg(e^z) &= ln|e^w| + i\cdot Arg(e^w) \phantom{\iff} \\
\iff && z + i2\pi n_1 &= w + i2\pi n_2 \\
\iff && z - w &= i2\pi(n_2 - n_1)
\end{flalign*}
\end{document}
The use of \phantom{\iff}
is meant to centre the longest part of the equation. \phantom{<stuff>}
sets a box the width of <stuff>
without actually typesetting anything.
The addition of showframe
is to highlight the text block boundary/margins, and is not needed in your final document.
For more information on AMS math environments (like flalign
used above), consider reading Herbert's mathmode
document.
\iff
flush left, with the rest of the equation aligned "as usual".