I am in the process of picking out my first DSLR. I have decided that the xsi is probably the right camera for me, but as we all know, the kit lens is not very good. Although I would still buy the entire kit, buying another lens has been recommended. I am looking at the B&H website. Other people on here recommended a 50mm 1.8. I also see on the website a 50 mm 1.4 and a 50mm 2.5. What exactly do the numbers mean? What is the difference between a 1.4, 1.8, and 2.5?