But wait—if Fresnel lenses do away with the bulging curves of traditional lenses, why are these lenses curved?

Turns out, Oculus went a step further down the optical tunnel, and manufactured hybrid Fresnel lenses.

These lenses are made up of concentric prisms of uniform thickness (like any other Fresnel lens). But an extra, sloping layer is added to the back of the lens, creating a shape that mimics a traditional curved lens—and allowing the focus to vary along the vertical axis of the lens.

That's why the Rift instructs you to focus the headset by simply pushing it higher or lower on your face. Look through a different part of the lens until you find the focus suited to your eyes. Bingo, no more three-sets-of-lenses Rifts.