You vary the exit angle of the lens assembly to make a larger image circle and can move the lens further away from the focal plane. However, Canon EF lenses all focus a exact distance from the flange of the lens to the sensor. You will see lenses inset varying amounts as part of the compromise of distance / exit angle. Look into the rear of your lenses, the distance to the glass often varies. And, on some designs, that rear element is part of a focus group, and it moves at the same time different elements are moving to compensate and adjust things like exit angle.
Optical lens design is the stuff of PHD's and many years of experience. There is no formula to design a lens, its trial and error. Obviously, you can start with a existing design and tweak it, but in the end, its still ray tracing. This used to be done on paper, tracing light rays as they travel thru a lens, then making a adjustment, and tracing again, maybe thousands of times.
Now computers do this, but since the variables are almost infinite, discovering a new optical formula that is better than existing ones, and does not cost a fortune to make is extremely difficult. That's why Canon and others patent a optical design or formula when they discover one that shows promise.
I'm not a optical designer, but have subcontracted lens design for my project to another company, and watched the process over the two years it took to complete. They did the ray tracing manually on paper, because it was a relatively simple lens. The Chief designer had a PHD, but had to consult with other experts frequently to get the job done. The final product managed to exceed requirements, which amazed everyone involved, since industry experts told us that what we wanted couldn't be done. It was also very expensive, there were many dozens of the lenses in each of our products.
http://en.wikipedia.org/wiki/Optical_lens_design
