I'm at the beginning of my search for a new lens. We'll be leaving for Alaska in a few weeks, and I think I'll be frustrated with my current lenses when it comes to shooting wildlife and glaciers. My current equipment is a Canon 40D, and the lens I use most often is the EF 24-105mm f/4L IS USM. I tend to be good about using a tripod when I shoot, but when I'm in Alaska I wonder about the practicality of using a tripod while trying to shoot moving animals, whales, calving glaciers, etc. The guy at the photo store recommended to me the Sigma 150-500mm f/5-6:3 AF APO DG OS HSM Telephoto Lens. He said it would be relatively light for a telephoto, and it can be hand held quite successfully. I do plan to take my camera and try it out. But the research I've done on it says it's not the best in low light. If it's gloomy in Alaska, it may not be the best. Do you have any other suggestions for lenses to inquire about? Sigma versus Canon? I think I'd prefer to stay with another Canon 'L' lens, but I don't think they have anything similar in zoom range to the Sigma. I own the Canon Zoom Lens EF 75-300mm 1:4-5:6, which I'm considering trading in for something else. I've never been crazy about that lens. After buying my first 'L' quality lens from Canon (mentioned above), I've found this 75-300 lens disappointing. Now having said all of this, I do want to mention that I plan to upgrade cameras soon, but I do not have time to adequately research this before our Alaska trip. So I'll stick with the 40D for now, but I do want to be sure that any lens I buy now will work on an upgraded camera (Canon). I hope I've given you enough information to go on. Please give me your suggestions for capturing the best of Alaskan wildlife and nature shots. Thanks!