I have heard that a good lens for taking portrait-like photos is to get a 50mm lens. I currently have a 28-135mm lens for my Canon 40D. Would I benefit in using a 50mm lens for taking pictures of my 12 month old son? If the 50mm is the way to go, is there a BIG difference in the 1.8 vs. 1.4? The f/1.4 seems to be 3x the price of the f/1.8. Is the cost difference worth it? I am by no means a professional photographer, I just enjoy taking pictures. Thanks for your input.