How do I reverse-mount a Canon 50mm f/1.4 for macro on a Rebel XTi?

To mount a lens backwards directly on your Rebel XTi, you need a reverse mount adapter (also called a reverse ring) with an EF camera mount on one side and your lens’s filter-thread size on the other. It attaches to the camera body, and the lens mounts to it in reverse.

A coupling ring is different: it lets you mount one lens in reverse onto the front of another lens, which can produce even higher magnification.

A few practical notes:

• Expect very shallow depth of field at macro distances.
• Focusing can be slow and finicky; you usually focus by moving the camera/lens rather than using the focus ring.
• With a reversed lens, aperture control is awkward. On lenses like the Canon 50mm f/1.4, you generally need to set the aperture while the lens is mounted normally, then reverse it.

So for your stated goal, start with an EF reverse mount adapter sized for your 50mm lens. If you later want more extreme macro, look into a coupling ring.