[Tex/LaTex] \includegraphics optional argument not resolved if macro

Question

\documentclass[a4paper,10pt,twoside]{book}
\usepackage[demo]{graphicx}

\begin{document}

\newcommand{\scalefactor}{scale = 0.25}

\begin{figure}[!ht]
  \centering
  %Next line does not work
  \includegraphics[\scalefactor]{image}
  % The following line works
  %\includegraphics[scale = 0.25]{image}
\end{figure}

\end{document}

Output:

! Package keyval Error: scale = 0.25 undefined.

I need to scale a number of images by the same factor, hence the command.

1. How to solve this and more importantly
2. Why is this only specific to \includegraphics

Answer 1

Try this way:

\newcommand{\scalefactor}{.25}
\includegraphics[scale=\scalefactor]{image}