Depth of field is relative...while f/7.1 will give you more DOF then f/1.8, at a close focus distance to your subject, you will still get a short distance that is in focus. Check out this on-line DOF calculator:
http://www.dofmaster.com/dofjs.html
I plugged in Canon 10-40D line, 50mm focal length, f/7.1, and a distance of 5 feet, and got these results:
Depth of field
Near limit: 4.63 ft
Far limit: 5.43 ft
Total: 0.8 ft
On the other hand, using the same settings, with a subject distance of 50ft, you get:
Depth of field
Near limit: 27.4 ft
Far limit: 282.2 ft
Total: 254.7 ft
Given your distance to the subject, even at f/7.1, you will only have a relatively narrow band that is in focus.