The following table has unwanted right and left white spaces. But you cannot see them because the figure has been converted to PNG with alpha channel enabled. 

How to trim the left and right white spaces when a table is previewed?

![enter image description here](https://i.stack.imgur.com/ZAIlf.png)

```
\documentclass{article}
\usepackage{siunitx}
\usepackage{booktabs,longtable,array}
\usepackage[active,tightpage]{preview}
\PreviewBorder=0pt
\PreviewEnvironment{longtable}

\begin{document}
\begin{longtable}{*3{m{\dimexpr0.5\linewidth/3\relax}}}
        \toprule
        Unit & Macro & Symbol\\
        \midrule
        ampere & \verb|\ampere| & \si{\ampere}\\
        candela & \verb|\candela|& \si{\candela}\\
        kelvin & \verb|\kelvin|& \si{\kelvin}\\
        kilogram & \verb|\kilogram|& \si{\kilogram}\\
        meter & \verb|\meter|& \si{\meter}\\
        mole & \verb|\mole|& \si{\mole}\\
        second & \verb|\second|& \si{\second}\\
        \bottomrule
\end{longtable}
\end{document}

```

Note that I am not talking about the white spaces that can be removed by `@{}`s. The unwanted white spaces can be understood if you compile my MWE.