Thank you so much for pointing me towards that book. I suppose what I was overlooking was that the image area over the source area is really the same thing as d^2(θ)/(dβ)^2 . I think I am on the right track now.
EDIT: Hmmm... actually I am not quite following the logic here. If mu =...
In gravitational lensing, the image magnification is defined as the image area over the source area. But many texts also give it as the inverse of the determinant of the jacobian, A, of the of the lens equation. My question is how these are equivalent.
The lens equation is...