So I got a cheap lens (Phoenix 19-35mm f/3.5-5.6) at a thrift store, and also picked up a Tamron 24-80mm f/something (don't recall right now) Was going to keep only one, but would like to do a ...

You can use it to test your camera with different lenses, especially for close-up lenses that are used to shoot small items. A standard close-up lens on a full-frame SLR usually has a 1:1 ratio ...