focal lenth is the same.
so there is no fx and fy.
But in Cali, they do exist.
That is not focal lenth. It measures the difference between vertical and horizontal pixel density.
We hope every pitch of minimal basic pixel looks like a perfect square.
But in reality it not even along vertical and along hori.
f*ax=fx,
f*ay=fy.
ax represents num of pixels along x axis.
ay represents num of pixels along y axis.
原文：http://www.cnblogs.com/stevenxiu/p/5623275.html