How to limit the figure caption width?

The caption package provides many enhancements: for example

\usepackage[margin=1cm]{caption}

Reducing the caption to the figure width seems not a good idea in general, particularly if a couple of figures are on the same page or on facing pages. On the other hand reducing the width and maybe the font size can be a good typographical device to isolate them better from the context.

in caption package guide:

Only fixed widths are supported here; if you are looking for a way to limit the width of the caption to the width of the figure or table, please take a look at the floatrow[8] or threeparttable[22] package.

\documentclass{article}
\usepackage{floatrow}
\begin{document}

\begin{figure}
\ffigbox[\FBwidth]
{\caption{caption text caption text caption text caption text 
caption text caption text caption text caption text 
caption text caption text caption text caption text }\label{...}}
{\rule{50mm}{20mm}}
\end{figure}

\end{document}

I also encountered the problem. Sometimes I use therefore two minipages to put two figures beside each other, so not much vertical space is lost.

\documentclass{article}
\usepackage{caption}
\begin{document}
 \begin{figure}
   \begin{minipage}[b]{0.6\textwidth}
     \centering
     \rule{0.4\textwidth}{3cm}
     \captionof{figure}{%
       Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
       Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis.
       Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna.
     }
     \label{fig:a}
   \end{minipage}\hfill
   \begin{minipage}[b]{0.3\textwidth}
     \centering
     \rule{0.8\textwidth}{3cm}
     \captionof{figure}{%
       Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
       Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis.
     }
     \label{fig:b}
   \end{minipage}
 \end{figure}
\end{document}