30D focus problems

Hello to all. Today is my first day with this forum, so please forgive any mistakes I may make. I am fairly new to photography. I have a Canon 30D with two lenses: 24-70 1:2.8 and a 72-200 1:2.8. My problem is with the focus. On all focus The camera randomly chooses where to focus. When I press the shutter button half down the focus will change but not necessarily. I want the whole frame to be in focus but cannot achieve this. What am I doing wrong? When taking a succession of photos of the same subject or scene the focus could change between shots. Very frustrating! HELP

Welcome to POTN....stand by for lots of answers. First and foremost...have you taken some time to familiarize yourself with the controls and operations of the camera? Reading the manual for the camera and the lenses will unlock a lot of secrets. The 24-70 is a fantastic lens, but it does require some practice to master. Depth of field at f/2.8 is very shallow....so there is going to be little margin for error when it come to having the subject in focus...stopping the lens down to f/8 will give you much more DOF...but you will sacrifice shutter speed in the process.

Experiment....take lots of images....and then post the results here that highlight the problems you are experiencing....makes it much simpler for us to give you a comprehensive answer.