Using the Kenko 12mm tube on the kit lens
Images were reduced in size but not cropped. Conversions from RAW were made at same settings. Images were all shot aperture priority (Av) at f/10 using the Kenko 12mm tube which had been ground out inside to allow seating of the EF-S lens rubber bumper.
The right image was shot with focus set on the MINIMUM distance available on the lens (.28m/.9ft). The others were shot with the lens set on infinity. Attempts to shoot the entire set using the minimum focus distance failed because the point of focus of the lens zoomed below about 28mm is INSIDE the lens. The lighting of the 18mm shot is 'different' because the point of focus is about 1mm from the front glass and inside the shadow of the lenshood.

Opinions: Do not buy extension tubes to use with the kit lens because the smallest available tube (12mm) is too long to be of practical use. If you are going to buy tubes, also buy the 50mm f/1.8 lens (half the cost of the tubes) with works well even with all three tubes. The tubes also work well with the 70-200 zoom (I have this and the 50mm on other pages) or other longer lenses. Modifying the 12mm tube is possible (I did it) but is a total waste of effort and risk of error. The modified tube should work better on the 17-85 EF-S (at longer focal lengths) but don't even think of trying this with the 10-22!!!! Retrofocus design wide angle lenses are simply not made for macro use.