I'm reading Cook's paper "Stochastic Sampling and Distributed Ray Tracing", I don't understand how the rays are generated. He says:

Determine the focal point by constructing a ray from the eye point (center of the lens) through the screen location of the ray. The focal point is located on this ray so its distance from the eye point is equal to the focal distance.

I can't parse this image. Where is my screen plane located according to this image? What is the focal point? If you see some optics picture of a thin lens, the focal point is always in the same place, where all the rays converge. If it is always the same point, why do have I to calculate it for each ray?