The kerning option of microtype

Question

I wonder what the kerning option of the microtype package does? In the manual it says that kerning=true will give additional kerning. But how? Is this something that has to be specified in addition to activating the option?

I can see no visual difference or any difference in filesize when compiling the two following examples with pdfTeX-1.40.10. Example 1:

\documentclass{article}

\usepackage{lipsum}
\usepackage[kerning=true]{microtype}

\begin{document}

\lipsum[1-5]

\end{document}

Example 2:

\documentclass{article}

\usepackage{lipsum}
\usepackage[kerning=false]{microtype}

\begin{document}

\lipsum[1-5]

\end{document}

Answer 1

kerning is not a "true-false" option. Its purpose is to add some kerning independently of the adjacent letters, for example to set the French semicolon that needs a space before it. However it's not working well: for example, the colon requires a normal interword space before and after it, and this space should stretch or shrink along with the other spaces on the same line, which is impossible to get with kerning.

See section 5.4 in the manual of microtype.