Lean On Technology
In 1968, using the Canon FT QL, on the left, I had to learn to focus using only my eyes and to learn the meaning of Depth Of Field, the portion of the area in front of me that would be in focus in the photograph. These days, using the Canon EOS 5Ds, on the right, I can let the wonderful technology of Auto-Focus do the focusing for me. Yes, I will almost always be in the manual mode but for focusing, especially on the fly, I love the Auto-Focus feature. Canon's version has a variety of settings but I typically stay on the "all purpose" setting. I can also predict my DOF using one of the many applications that will calculate it if I enter the distance, focal length, and f-stop.
If you're in the learning phase you should try to get a feel for your settings before letting the technology take over. If you're in a situation, as I usually am, in which getting the shot before it disappears is critical, technology will be a big help.