First I read that you want a full size 5D sensor when you shooting with a fish eye lens. The smaller sensor on a 50D wont do as well. When it comes to fisheye lenses, in order to get a fuller barrel effect you want a more panoramic view. Something to do with the sensor sizes the sensor of a 5D does much better for this effect. 50 d Sensor Size: 22.3×15 mm 5d Sensor size, 36 x 24 mm .... <---much better for panaramic fisheye barrels effects. But then I read if you use a specific lens engineered or designed for a a 50D sized aps sensor it'll eliminate this problem. Meaning. When debating this topic, people are putting more attention on a specific lens. Their favored or more popular fisheye lens is interchangeable between 5D and 50D but works much better on the 5D cuz of the larger sensor. <-I could be wrong. Im working on figuring this all out. So what if I purchase a fisheye lens specifically made for 50D aps sized sensor. Will I get the same barrel effect as the 5D? Or.. Will it improve the barrel effect enough? That I shouldnt feel the need to upgrade from a 50D to a 5D? Also, Im led to believe 2.8 faster glass does two things that are important. Much better for shooting in low light situations but the 2.8 allows for stronger blurred backgrounds? If Im using this lens mainly for weddings party shots and dance floor shots. Do I need a 2.8 or will a 4.5 work?