equipment recommendation - How to decide which reversing ring is suitable for a lens?

Nikon's website doesn't seem to suggest any reversing rings that go with specific lenses (in my case a 50mm f/1.8G) .

For macro shots how to decide which reversing ring goes with it? Yeah, Amazon shows lots of results, but on which basis should I decide what?

Answer

You base a reversing ring on the filter thread size and mount for your camera. To determine your size, check this question. That particular lens has a 58mm filter size, so you need something like this that uses the Nikon mount.

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_{I'm assuming you mean the 50mm f/1.8 G and not the D, since your D3100 won't autofocus with the D but will with the G. If you mean the D version, its 52mm.}