documentclass{article}

usepackage{graphicx}
usepackage{subcaption}

newlength{figwidth}
newlength{figheight}
newlength{figdim}

begin{document}

begin{figure}[htbp]
  begin{subfigure}{0.59linewidth}
    % Length
    setlength{figwidth}{linewidth}
    setlength{figheight}{5cm}
    settoheightfigdim{includegraphics[width=figwidth,height=figheight,keepaspectratio]{example-image-a}}
    setlength{figheight}{figdim}
    % Figure
    centering
    includegraphics[width=figwidth,height=figheight,keepaspectratio]{example-image-a}
    caption{thefigdim, thefigheight, left}%
    label{fig:left}
  end{subfigure}%
  hfill
  begin{subfigure}{0.39linewidth}
    % Length
    setlength{figwidth}{linewidth}
    % Figure
    centering
    includegraphics[width=figwidth,height=figheight,keepaspectratio]{example-image-b}
    caption{thefigdim, thefigheight, right}%
    label{fig:right}
  end{subfigure}%
  caption{Test}
  label{fig}
end{figure}

end{document}