Hi. I've been staying in Wrangell - St. Elias Park, Alaska until late September. I'd like to photograph wildflowers in this area in addition to landscape and wildlife. Since I don't have any macro lenses, I'd like to add an extension tube. I use Canon 5D Mark II + 24-105mm, 17-40mm, 70-300mm, 100-400mm. I've never used any extension tubes before... I looked at Canon's extention tubes and Kenko's set (12, 20 & 36mm). It seems like the Kenko's set is a good value. Does any one of you used either Canon's or Kenko's? Do you recommend one over another? Please share your opinion.