margins in beamer columns

Question

I have some trouble with margins in multi-column layout beamer slides. If put the following snippet in a non-rounded theme like "Rochester", you will see that

1. the boxes of the first row have different margins than the boxes of the 2nd row (the 1st row has an "indent", which also affects the position of the central line between the boxes)
2. the rightmost margin differs from the leftmost margin (the left margin is smaller than the right one, which optically goes away when one uses shadows)

any ideas?

\begin{frame}[t]
\frametitle{...}
\begin{columns}[T]
  \column{.50\textwidth}
  \begin{block}{...}
     ...
  \end{block}
  \column{.50\textwidth}
    ...
\end{columns}
\vspace{3mm}
\begin{columns}[T]
  \column{.50\textwidth}
  \begin{block}{...}
    ...
  \end{block}
  \column{.50\textwidth}
  \begin{block}{...}
    ...
  \end{block}
  \end{columns}
\end{frame}

Answer 1

How should this work when you define two columns each with a width of 0.5\textwidth. You have to add the columnsep and all together is > \textwidth, the reason why beamer shifts all to the left.

\documentclass{beamer}
\usetheme{Rochester}

\begin{document}

\begin{frame}[t]
\frametitle{...}
\begin{columns}[T]
  \column{.425\textwidth}
  \begin{block}{...}
     ...
  \end{block}
  \column{.425\textwidth}
    ...
\end{columns}

\noindent
\rule{\textwidth}{5pt}

\begin{columns}[T]
  \column{.425\textwidth}
  \begin{block}{...}
    ...
  \end{block}
  \column{.425\textwidth}
  \begin{block}{...}
    ...
  \end{block}
  \end{columns}
\end{frame}

\end{document} 

Look into the documentation for the value of the columnsep; then you can define it exactly to \textwidth.