I recently purchased a Sigma 24-70mm f2.8 lens which I am using on a Canon 40D body. Around 8 of 10 shots were soft, regardless of subject matter. Exposure, white balance etc. all looks fine. I only shoot in manual mode. After doing some research and a professional friend taking some pics (also soft) I took the lens and camera body in and had a firmware upgrade.
Since the upgrade I have seen a huge improvement, however, when it comes to portraiture many of my shots still look soft. This is hampering me enormously as I am trying to go pro, but cannot put my name out with this "track record". I have noticed that distance from subject makes somewhat of a difference (further away = softer). Low light is also problematic despite using a flash. Another friend says Sigma lenses are apparently known to have this "soft" problem. What should I do?