Reduce horizontal space between subfigures

Question

I have created a 2x2 subfigure with the following code:

\begin{figure}
\centering
\begin{subfigure}{0.49\textwidth} % width of upper left subfigure
    \centering
    \includegraphics[width=.5\linewidth]{Test2/22_macro.png}
    \caption{Sample 2.1 - \SI{20}{ppm}} % subcaption
\end{subfigure}%
\hfil
\begin{subfigure}{0.49\textwidth} % width of upper right subfigure
    \centering
    \includegraphics[width=.5\linewidth]{Test2/23_macro.png}
    \caption{Sample 2.3 - \SI{50}{ppm}} % subcaption
\end{subfigure}%
\vfill
\begin{subfigure}{0.49\textwidth} % width of lower left subfigure
    \centering
    \includegraphics[width=.5\linewidth]{Test2/26_macro.png}
    \caption{Sample 2.6 - \SI{100}{ppm}} % subcaption
\end{subfigure}%
\hfil
\begin{subfigure}{0.49\textwidth} % width of lower right subfigure
    \centering
    \includegraphics[width=.5\linewidth]{Test2/27_macro.png}
    \caption{Samples 2.7 - \SI{150}{ppm}} % subcaption
\end{subfigure}%
\caption{} % caption for whole figure
\label{}
\end{figure}

But as you can see from the image there is a lot of white space between fig a and b, and fig c and d.

How can I control the horizontal distance between subfigs more effectively?

thanks in advance.

Answer 1

from your question, subfigure width roughly equals half of \textwidth and image width equals half of subfigure width. So, you can put all four sub-images in one row (first figure), or maybe in two rows (second figure).

Latex code for both cases:

%%%%%%%% first image
\begin{figure}
\begin{subfigure}[b]{0.24\textwidth} % width of upper left subfigure
    \includegraphics[width=\linewidth]{Test2/22_macro.png}
    \caption{Sample 2.1 - \SI{20}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.24\textwidth} % width of upper right subfigure
    \centering
    \includegraphics[width=\linewidth]{Test2/23_macro.png}
    \caption{Sample 2.3 - \SI{50}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.24\textwidth} % width of lower left subfigure
    \includegraphics[width=\linewidth]{Test2/26_macro.png}
    \caption{Sample 2.6 - \SI{100}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.24\textwidth} % width of lower right subfigure
    \includegraphics[width=\linewidth]{Test2/27_macro.png}
    \caption{Samples 2.7 - \SI{150}{ppm}} % subcaption
\end{subfigure}%
    \caption{common caption} % caption for whole figure
\label{fig:foursubfig}
\end{figure}

%%%%%%%% second image
\begin{figure}
\begin{subfigure}[b]{0.49\textwidth} % width of upper left subfigure
    \includegraphics[width=\linewidth]{Test2/22_macro.png}
    \caption{Sample 2.1 - \SI{20}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.49\textwidth} % width of upper right subfigure
    \centering
    \includegraphics[width=\linewidth]{Test2/23_macro.png}
    \caption{Sample 2.3 - \SI{50}{ppm}} % subcaption
\end{subfigure}

\begin{subfigure}[b]{0.49\textwidth} % width of lower left subfigure
    \includegraphics[width=\linewidth]{Test2/26_macro.png}
    \caption{Sample 2.6 - \SI{100}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.49\textwidth} % width of lower right subfigure
    \includegraphics[width=\linewidth]{Test2/27_macro.png}
    \caption{Samples 2.7 - \SI{150}{ppm}} % subcaption
\end{subfigure}%
    \caption{common caption} % caption for whole figure
\label{fig:foursubfig}
\end{figure}

%%%%%%%% third image
\begin{figure}
\centering
\begin{subfigure}[b]{0.25\textwidth} % width of upper left subfigure
    \includegraphics[width=\linewidth]{Test2/22_macro.png}
    \caption{Sample 2.1 - \SI{20}{ppm}} % subcaption
\end{subfigure}%
\quad
\begin{subfigure}[b]{0.25\textwidth} % width of upper right subfigure
    \centering
    \includegraphics[width=\linewidth]{Test2/23_macro.png}
    \caption{Sample 2.3 - \SI{50}{ppm}} % subcaption
\end{subfigure}

\begin{subfigure}[b]{0.25\textwidth} % width of lower left subfigure
    \includegraphics[width=\linewidth]{Test2/26_macro.png}
    \caption{Sample 2.6 - \SI{100}{ppm}} % subcaption
\end{subfigure}%
\quad
\begin{subfigure}[b]{0.25\textwidth} % width of lower right subfigure
    \includegraphics[width=\linewidth]{Test2/27_macro.png}
    \caption{Samples 2.7 - \SI{150}{ppm}} % subcaption
\end{subfigure}%
    \caption{common caption} % caption for whole figure
\label{fig:foursubfig}
\end{figure}

Edit: You can also have small images in two rows (third figure).