hi, i have a canon rebel xti and i've started using the sigma 18-200mm f/3.5-6.3 lens. i've tried taking photos with a low fstop number to produce a shallow DOF. but it hasn't been working. the background is not blurred as i had hoped. i don't understand what's wrong. here are some examples of the photos i took recently with their respective fstop and shutterspeeds 1) http://www.flickr.com/photos/rebeccathecurious/424669627/ f/3.5, 1/1600 sec 2) http://www.flickr.com/photos/rebeccathecurious/424636546/ f/3.5, 1/3200 sec any ideas as to what i'm doing wrong would be appreciated. thanks!