Try this:
From
http://www.learnslr.com/cs-canon-50d-guide-faq/11-50d-basic-features
AI Servo / AI AutoFocus / One Shot
A quick press of the Focus Mode button on the top of your 50D will allow you to use the top dial to quickly change from One shot to AI Focus, or AI Servo mode.
1. One Shot: Canons default focus mode, the camera will not take a photo until the camera locks onto its subject matter. Once the red light in the view finder lights, the photo can be taken. Appropriate for non-moving subject matter, portraits, landscapes and other NON-moving subjects.
2. AI Servo: The 50D constantly refocuses attempting to calculate the moving subjects next movement and based on that prediction focus the lens on the subject. This mode should be used when shooting sports, moving objects, or any type of image where the subject matter is constantly changing location. This mode can be combined with continuous shooting to capture 6.3 frames per second while following a subject. If the auto focus is able to perform adequately, you will get 6.3 frames per second in focus while your subject matter is moving.
3. AI Auto focus: Mode where the 50D determines whether One Shot or AI Servo should be used. In general this is not a good mode to be in, and I avoid it at all times. Its quite quick and easy to switch between one shot and AI Servo do that when switching is necessary.
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I'm thinking for the narrow depth of field pictures that you are taking (Either by virtue of a large aperture, shallow DOF due to shortened distance of subject) you should be in one shot focus mode.
Do the Canon people agree with the Nikon Mom???
I'd really like to help Jenseib get a handle on her problem before she leaves on her trip....
Good luck!
Marlton Mom