There's two flavors of reversing rings: 1. Reversing a single lens on the camera body.
Biggest problem with this method is controlling the aperture on today's lenses that don't have an aperture ring.
Option 2: Reversing one lens in front of another.
Again, controlling the aperture on the reversed lens may be an issue, but it only needs to be opened to maximum. This is accomplished in several ways, depending on your camera manufacturer.