This is not the case in using real world lenses. Since a
lens has a spherical figure, light coming through near the
edges of the lens, more or less, tends to be distorted.
There are two types of distortion parameters: (1)
diagonal distortion, and (2) XY crosswise distortion.
If you want a perfect spherical distortion, set "Diagonal"
value at 10 and "XY" value at 0. Otherwise,
choose any value between 5 and 20 for "Diagonal"and
a value between -40 and 40 for "XY", and
explore the result.
Proceed to the textscroll
menu if you have checked the textscroll box; otherwise
go to the expert menu.