Shortly after I got my Kenko Extension tubes, I read something on the internet suggesting that combining the extension tube with a wide angle lens (such as the EF Canon 16-35 f/2.8 L II) would allow the photographer to take a picture of the back side of the front element of the lens. Some of you may think this sounds silly. There may even be some good reasons for it to sound silly. What does the back side of the front element look like? How do you know if it’s in focus? Even with out these answers, I set out to do a little experiment. How much extension tube could I use before I couldn’t get the lens any closer to the subject (physically).
I decided to take a series of images starting with just the EF 16-35 f/2.8 L II, and then successively adding extension tube. I started at 35mm and no extension tube. Next I added the 12mm extension tube. Then the 20mm, then the 36mm. Finally, I opened the lens up to 16mm. Since my subject was a flat LCD display, and the front element did not stick out past the lens-hood mount, I went ahead and touched the lens to the display in an effort to get the image in focus. As you can see, I did not succeed. Not even close.
It would not suprise me if there was a rule that said you can’t put an extension on lens that is longer than the lens and have it function. That said, the 36mm extension did work on the 16-35 @35mm, although it would not be practical outside of a highly controlled environment. The lens/subject distance is less than 1/4in.
Things I noticed during the test
I did notice during this test, that the EF Canon 16-35 f/2.8 L II, under macro-ish conditions, shows the issues of soft(er) edges and sharper center more acutely than non-macro conditions. I have not noticed any noticeable softness with any other images, but it is apparent with the extension tubes.