A footnote on page 12 of the subfig documentation says in part:
Note that the ‘width’ may be larger than the width of the sub-float body, or equivalently, the ‘width’ may be negative to provide more room for a caption if the sub-float body is small. The box holding the caption is centered relative to the sub-float body.
I must not be understanding something.
First, by looking at Figure 91 on page 35, don't they mean to say that you can set the margin to a negative value in order to get a caption that is wider than (and centered relative to) the sub-float body?
Even if this is right, I can't get a simple example to work. See my MWE below.
What am I doing wrong?
MWE:
\documentclass{article} \usepackage{caption} \usepackage{subfig} \begin{document} \begin{figure}[ht] \centering \subfloat[very long subfloat caption]{ \framebox[3cm][c]{ subfloat body } } \end{figure} \begin{figure}[ht] \centering \captionsetup[subfloat]{width=-2cm} \subfloat[very very long subfloat caption]{ \framebox[3cm][c]{ subfloat body } } \end{figure} \begin{figure}[ht] \centering \captionsetup[subfloat]{margin=-2cm} \subfloat[very very long subfloat caption]{ \framebox[3cm][c]{ subfloat body } } \end{figure} \end{document} 
subcaption, anyway.