The contextrefmanual tells me in table 3.2 the following on the behaviour of the variables denoting the width of texts:

[cid:D468CE47-C987-414C-84B0-47652AC59361@fritz.box]

But the claim that \textwidth gives the width of a column is false. See the following minimal example, the first picture is set with 0.45\makeupwidth, the second with 0.9\textwidth.
It is clear that \textwidth does not follow the claim.

Hans van der Meer

ConTeXt  ver: 2013.01.27 21:24 MKIV  fmt: 2013.1.28  int: english/english

\setupcolumns[rule=on,n=2]
\starttext
\startcolumns
\input tufte
\placefigure{Test picture}{\externalfigure[test.jpg][width=0.45\makeupwidth]}
\placefigure{Test picture}{\externalfigure[test.jpg][width=0.9\textwidth]}
\input tufte
\stopcolumns
\stoptext

[cid:EB7A4C53-E754-481F-AB87-D6DE646347FD@fritz.box]

Hans van der Meer