[OS X TeX] Problem with setting the captionfont space

[Sven-Oliver B]

I want to set my Caption, despite of the rest of the document, which  
is "onehalfspacing", on "singlespacing".

using:

\usepackage{setspace}
\onehalfspacing


\usepackage{caption}
\DeclareCaptionFont{singlespacing}{\singlespacing}
\captionsetup{font={singlespacing}}

doesn't work at all. Why?

My System
G4, Texshop

Thanks

Sven-Oliver B.
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