Canon 5d Mark iii Help with focusing

The problem with the dof preview on modern dslrs is that it doesn't accurately depict the actual dof through the viewfinder. The only way to get an accurate depiction of dof is to use the dof preview button in live view, which isn't practical for many shooting situations. And even then, what appears sharp on your camera's lcd may not turn out that way when viewed at full size on your computer screen.