Hi Zemax Community,

I am reading the book,* introduction to lens design with practical zemax examples.*

And in the chapter 11.6, it writes

*“ We now combine spherical aberration and defocus mathematically, so that we can find the position of the minimum blur(MB) spot*

* W = W040*ρ^4 + Wd*ρ^2 *

* this occurs when*

*Wd = -3/2* W040 “*

I just wondered

- Why Wd = -3/2*W040 makes the MB minimum ,how to get the -3/2?
- is zemax using this calculation in M solve?

Thanks in advance

Best regards

Yang

**Best answer by Akhil Dutt Vijayakumar**