[ptx] why f(r) mixed symmetry?

[Klaus Foehl]

Hello,

A question probably best to ask Helmut Dersch if he were still active...

Why are parameters a, c in the lens formula f(r)= r*(1+a*r+b*r^2+c*r^3) ?
With the rotational symmetry of almost all lenses and an analytical ansatz
shouldn't the function f be point symmetric, i.e. -f(-r)=f(r) and thus
only containing odd powers of r?

How difficult would it be to add a parameter d*r^5 into the
hugin/nona/Panotools framework? (Yes I know "d" is already used up)

[and I see fudge parameters useful when photos are not parallaxe free]

Klaus