I have a Sigma 70-300 f/4-5.6 APO DG Macro lens and a Canon 100mm f/2.8 USM Macro. I'd love to be able to get really good close up macro shots and also some great bird shots. The Sigma doesn't get me close enough to the birds. Can I use an extension tube on the Sigma (and if so, what kind would fit that lens) for better bird shots and a close-up lens on either for really close macro shots? I saw someone shooting spiders who was using 2 close-up lenses on his camera (it was a Panasonic, I believe). I have a Canon xti. Of the two, I would prefer to get better bird shots. I'm sure I can't afford to get both an extension tube and a close-up lens right now.