I was trying to shoot with my 430ex flash a while back, and then I realized, I forgot how to set the shutter speed in manual mode on my 50D.  :unamused:

I found this cool site that lets you calculate Depth of Field based on the lens parameters you punch in.  You can see how certain combinations will give you very tight limits. You just have to move out of position by a fraction of an inch to throw off the focus on your subject.