Thanks by the answer, but I had already thought in this. I would like something more definitive.
By the way, at http://wiki.contextgarden.net/Reference/en/setupcaptions I can see a command argument. I was wondering if a custom command can do the trick, but I don't know how I should use this arg and what it is for.
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Wagner Macedo
I don't know if I get you right, but I think you are talking about
the width of the caption. It adaptes to the with of the figure.
There is the “minwidth” parameter for \setupcaptions, but since the
short captions will not be centred it's probably not what you aim
for. I don't have a solution, but a workaround that alters the
global width setting for an individual caption: