Figure textwidth and badbox

Question

I am using a documentclass requried by a journal, which also defines the format of a picture. I am using the following code for expanding the figures to the page width and getting badbox error. What am I missing here?

\documentclass[12pt]{spieman}  
\usepackage{graphicx}

\begin{document} 
\section{Introduction}

\begin{figure}[b]
\begin{center}
\begin{tabular}{c}
\includegraphics[width=\textwidth]{pics/figures1.pdf}
\\
\hspace{1.6cm} (a) \hspace{7.4cm} (b) \hspace{2cm}
\end{tabular}
\end{center}
\caption 
{ \label{fig:figures1}
figure caption.} 
\end{figure}

\end{document} 

Thanks for the help.

Answer 1

You're not taking into account the padding around text at the beginning and at end of the column; you can remove that space using @{} (notice that I removed the center environment since is not really needed and adds extra vertical spacing):

\begin{figure}
\begin{tabular}{@{}c@{}}
\includegraphics[width=\textwidth]{example-image-a}\\
\hspace{1.6cm} (a) \hspace{7.4cm} (b) \hspace{2cm}
\end{tabular}
\caption{\label{fig:figures1}figure caption.} 
\end{figure}

The use of tabular however is not really necessary, and you could simply use something like

\begin{figure}
\centering
\includegraphics[width=\textwidth]{example-image-a}\\
\hspace{1.6cm} (a) \hspace{7.4cm} (b) \hspace{2cm}
\caption{\label{fig:figures1}figure caption.} 
\end{figure}