Changing the caption font and line spacing at the same time

Question

I am currently trying to change the font size as well as the line spacing within the captions of my document and am attempting this by using the setspace and caption packages. As the line spacing as well as font size belong to the same setting (font=...) i'm unsure how to set both values. This code:

\documentclass[12pt]{article} 
\renewcommand{\baselinestretch}{1.5} 
\usepackage[labelfont=bf]{caption}
\usepackage{setspace}
\captionsetup{font=footnotesize}

\begin{document}
some text 
\end{document}

As well as this code:

\documentclass[12pt]{article} 
\renewcommand{\baselinestretch}{1.5} 
\usepackage[labelfont=bf]{caption}
\usepackage{setspace}
\captionsetup{font={stretch=1.0}}

\begin{document}
some text 
\end{document}

Both work individually but i can't manage to set my font as stretch=1.0 as well as footnotesize. For reference, my normal text size is 12 pt and the line spacing is 1.5pt and i would like my caption font to be size 10pt (footnotesize) and have a line spacing of 1.0pt.

Any help is appreciated!

Answer 1

As already mentioned in @Jasper Habicht comment, you can write:

\documentclass[12pt]{article}
\usepackage[font={small,stretch=1}, labelfont=bf]{caption} % <---
\usepackage[onehalfspacing]{setspace} 

\usepackage{lipsum}

\begin{document}
\lipsum[66]
    \begin{figure}[ht]
\rule{\linewidth}{4pt}
    \caption{\lipsum[1][1-2]}
    \end{figure}
\lipsum[67]
\end{document}

For details and all possibilities see caption package documentation.

Answer 2

Executing the low-level command

\renewcommand{\baselinestretch}{1.5}

in the preamble is almost always a mistake, as doing so also affects the line spacing in captions, tabular-like environments, and footnotes, which are generally supposed to be typeset single-spaced regardless of the line-spacing that's applied to the other parts of the document.

What to do? Since you load the setspace package anyway, I would like to suggest that you run

\usepackage{setspace}
\setstretch{1.5}

That way, you won't have to fiddle with the line spacing in captions.

Your main document font size is set to 12pt. To get the font size that's used in captions to equal 10pt, it suffices to run

\captionsetup{font=footnotesize}