[quote author=Fu-Kwun Hwang link=topic=1156.msg4456#msg4456 date=1245656734]
If you just need visual effect, you can change the width of shape (the GUI used to draw lens) and let it change with f.
[/quote]

i understand your suggestion,

any tips how i get get a suitable equation?

size X = size*2*(2- Math.sin(f)) doesn't look good
          = size*2*(1/f) is the correct relationship but it doesn't look nice.

i am stuck thinking of a eqn ;D