Extension tube test

When I got my Mamiya medium format, it came with three extension tubes. They are just these rings that you attach to the lens that basically extends the focal length of the lens allowing for closer than normal (or "macro") focusing. The results can be pretty cool, but they didn't come with instructions and I still don't really understand how I have to adjust the exposure. My photos keep coming out kinda dark. I think I am getting a little better, though. These are the results of my latest test.

My lemon tree finally has lemons on it!
I love how the light comes through our office window just before sunset.