Just accidentally discovered a trick with an extension tube. I had a 21mm extension tube on with my 18-55mm Canon kit lens, When I zoomed out to 18mm, I could suddenly see in-focus dust and bits of dirt in the blurred image. I was focussing on the front glass of the lens itself !!!
It was actually useful, as I can look through the viewfinder, watch while I clean it, and see if it is really clean or not. Kinda fun to watch the cleaning fluid spread over the glass and my big fingers approach with the cleaning paper to spread it around.