What DOF?

Tonight I was without a BF so after digging around in my photo equipment bags I decided it was time for more experimenting. I had gotten extension tubes some time ago and not messed around or tried anything much. You can see the curved surface of a lens housing really has just a fraction of an inch that is sharp that falls off to the left and right as the housing curves away. I tried a flat dime and it worked fine. The focus was super critical to adjust. Now to find an interesting subject.......