book1: update text around generating focused rays

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -2759,8 +2759,8 @@
 <div class="together">
 We don’t need to simulate any of the inside of the camera. For the purposes of rendering an image
 outside the camera, that would be unnecessary complexity. Instead, I usually start rays from the
-surface of the lens, and send them toward a virtual film plane, by finding the projection of the
-film on the plane that is in focus (at the distance `focus_dist`).
+lens aperture, and send them toward the projection plane (`focus_dist` away), where everything on
+that plane is in perfect focus.
 
   ![Figure [cam-film-plane]: Camera focus plane](../images/fig.cam-film-plane.jpg)