I am headed out to Yellowstone National Park in August. I am most interested in capturing some nice landscapes so I am looking to purchase a wide angle zoom lens. However I am also very interested in shooting weddings for which I also need a wide angle zoom for my Canon 50D. I own the Canon 24-70 2.8L but this lens is not wide enough on the 50D for weddings or landscapes. I have been looking at a Tamron 17-50 mm 2.8 which has good reviews. This lens give the equivalent of 27 mm with the 1.6 crop factor on the 50D. Is this wide enough to get some good landscapes at Yellowstone or do I really need an ultra wide angle starting at 10 or 11mm on the wide end. I've read a lot of websites and opinion is mixed on how wide is wide enough. The draw back of the ultra wide zoom besides cost is obviously not enough reach on the long end which will not very useful to me for weddings. Help!