I have a Nikon D50. I replaced the lens with a Tamron 28-75 f2.8. I like this lens, and it works for me. Recently I have been taking photos at the local speedway. The 2.8 is great for when it gets dark, but the 75mm is not long enough for the cheap seats. I was reading about the Sigma 80-300. The 300mm part is what I need, but the aperture is not. I was wondering what a teleconverter would do for me. How much aperture do you lose with one, and what would the focal length end up at. Say a Tamron or Kenko 2x. Possibly you have other suggestions? Would I be better off with the Sigma or a teleconverter? What i would like is a Nikon 20-200 f2.8, but this is just a hobby for me. I cannot spend even $800 for a used one right now. Ideas, anyone?