[NTG-context] basic fontsizes question

In the fonts book I found the \definebodyfont command, but it does not to do what I expected. What I need (for a poster) is a set of absolute font sizes. I tried the code below. I put some quirks in, like the huge value for "a", to see if it works, but it doesn't. It just uses the preset proportions relative to the preceding fontsize command (in the example, \tfx is bigger than \tf). \definebodyfontenvironment[palatino][24pt][ text = 24pt, small = 22pt, x=22pt, scriptscript = 16pt, xx = 16pt, big=28pt, a = 80pt, % 28pt b = 40pt, c = 50pt, d = 50pt, interlinespace = 3ex, em = italic ] \setupbodyfont[palatino,24pt] \starttext \tf tf\\ \tfa tfa\\ \tfb tfb\\ \tfx tfx\\ \tfx tfx \stoptext