For reverse-lens macro, the best choice depends on your goal:

• Highest magnification: use the 18-55mm at the wide end. Reversing a shorter focal length generally gives more magnification.
• Better image quality: the 50mm prime is usually the better option, since prime lenses often perform better than kit zooms for this kind of setup.

There are two common types of reversing rings:

1. Camera-mount reversing ring — attaches a reversed lens directly to the camera body.
2. Male-to-male filter coupling ring — lets you mount one lens reversed onto the front of another lens.

To choose the right ring, you need the filter-thread diameter of the lens you want to reverse, because reversing rings are usually specified by the lens thread size on one side and the camera mount on the other.

Reversing a lens works well for close-up subjects because lenses are normally designed for distant scenes; reversed, they can perform better at very close distances and also gain magnification from the added extension.