I have a Canon Rebel T3, and I'm very new to photography. I recently got a new close up filter that goes on top of my 18-55 lens (this question also applies with any lens I have). I hold the shutter button down to focus (AF mode) and when I have the picture I want, I press the shutter down completely to take the picture. Usually it takes the picture, however, sometimes instead of taking the picture, the camera wants to re-focus and usually blurs the picture. I've tried all the picture modes but no luck, and it can be quite frustrating. What am I doing wrong? How can I force the camera to take the picture when I want it to?
The problem is at such close distances, the depth of field is very shallow. That means that if you move the camera an inch, your subject will no longer be in focus, and the camera will try to refocus.
Many autofocus systems will "hunt" as they try to focus on close objects.
If you have a tripod, use it. The autofocus will probably work, as the camera will no longer be moving.
Are you using AI Servo or AI Focus mode for focusing? If so, you may want to try the One Shot mode, which will lock focus once you half-press the shutter.
But for macro, I would suggest not using autofocus at all. Change the camera to manual focus. Focus until the subject is more or less sharp, then stop using the focus ring, and simply rock back and forth, moving the camera towards the subject and then away to reestablish focus. In other words, with the lens focusing at a fixed position, move yourself and the camera back and forth until the subject is at that distance.
There are some tips for handheld macro focusing here: Are there macro focusing techniques for handheld shots?