I took my old Nikon Nikkor-HC 50mm f/2 and held it reversed set at f2 against the camera mount. The result is a rather simple macro lens. You can do this with almost any lens but a manual one is far better as you don't have to hold the aperture open. The downside is you have no focus and the depth of field is around 1-2 mm, you've got to kind of rock back and forth to get your subject in focus. While not the sharpest in the terms of detail the result is rather nice. Here we have a spider i found on a flower you notice just how shallow the dept of field really is as i couldn't get the whole spider in focus.