I read that you use the sigma 28-105 zoom lens. I have it and it sucks.
In dealing with the Sigma 28-105 lenses, remember that there is the 28-105 4-5.6 UC, the 28-105 4-5.6 UC II, and the just-released 28-105 3.8-5.6 UC III, as well as the 28-105 2.8-4. According to the test results, each new version of the 4-5.6 UC make was better than the last. I have the latest (UC III) and find it quite good for the "if you just have one lens" situations (just shot four rolls with it today). It has a minimum focusing distance of about 19", and does adequatly on an HTsi plus as far as focusing speed. If you have the old UC, you may be quite pleased with the UC III, and it is a lot less expensive than the 2.8-4 version. I would not recommend it over top quality prime lenses, however. It is, after all, an all-purpose zoom.