70 - 200mm L Lens question

[agentm]

Being new to lenses and still learning I want to know the following:

1. What is the difference between the Canon 70-200mm F2.8 L USM Lens and the Canon 70-200mm F4 L USM Lens?
I see the F-stop is different. How will that effect my photography? I see there is quite a significant price difference.

2. What does USM mean?


Thanks
Henk

[timmyquest]

USM: Ultra Sonic Motor, it is canons fastest/quietest auto focus motor..tis a good thing.

The difference in the 2.8 vs 4 f/stop is what adds the $600 (?) to the lens, yes it is the only difference, but depending on what your shooting it can mean a world of difference as it allows for a bit more light to enter the lens then the f/4 one...chances are you'll be plenty happy with the f/4 from what i've heard.

[Scottes]

Previous discussion a few days ago
http://photography-on-the.net/forum/showthread.php?t=22701&highlight=tripod+ring

[gillyworld]

The f2.8 IS lens is definitely worth the extra - if you were thinking of an f2.8 lens

gillyworld