Here I have my Sigma zoom 28-70mm 1:2.8 lens mounted on my Canon Rebel T3i (also know as the 600D). This first picture shows you how close I can get with the lens properly put on the camera.
This second photo will show a much closer view with the lens mounted on reverse. Huge difference!
I don’t know about anyone else but this is absolutely amazing to me because I can do macro photography right now and I don’t have to invest a lot of money at the moment!