I have extension tubes 1 and 2 and today I tried using tube # 2 for the first time. I tried it on the 180mm and 65mm lenses. In both cases, when looking through the finder, the focusing screen was almost totally opaque. I moved the bellows in and out, and tried it on objects at different distances and still got the same result. I waved my hand in front of the lens just to be sure that the shutter was open, and indeed it was.

I'd appreciate some guidance as to the use of extension tubes.