autofocus on rebel t3 doesn't work

[ncody530]

I can't take a picture in autofocus.  A flashing green circle appears in the lower right of the viewfinder and it doesn't take a picture.  I can take a picture in manual focus.  What is wrong?  I tried turning camera off and reinserting the battery and card and I tried going into the menu and clearing all camera settings - that didn't fix it.  

[Skirball]

It’s flashing because it can’t lock AF.  Are you trying to shoot in a dark setting?  Or too close to an object?  Or you have the AF on a single focus point and that point is on a blank/solid color.  If there isn’t contrast over a single point it won’t be able to find focus.  Make sure your camera is set to all focus points (button on the right that looks like part of a chess board).

[ncody530]

It won't take a picture anywhere - indoors outdoors no matter what I focus on and no matter what mode - auto, AV when the lens is set to autofocus.  It will only take pictures when the lens is set to manual focus.

[Skirball]

Do you have more than one lens?  If so, put that on and see if it's the lens or camera.  If not, I'd try to find someone with a Canon camera and try their lens.  Also, you can try cleaning the lens contacts (take the lens off) with a pencil eraser.  I've never had a problem with dirty contacts, and I'm rough I'm my cameras, but it's easy enough to try.   

If it's the camera, try doing a factory reset (in the settings menu).  If none of that works, send it in for service.

[TCampbell]

From Canon's page on understanding and correcting error codes:

"Warning: do not clean lens contacts on either a Canon lens or the camera body with an eraser! It’s incredibly easy to rub off the gold plating on these contacts, and end up with data communication problems, even if that wasn’t the original source of the problem! It’s obviously a good idea to clean the contacts if a lens communication error is reported, but our strong recommendation is to use a soft, clean cloth, perhaps moistened with isopropyl alcohol if you feel the contacts are smudged with fingerprint oil or similar debris and really need a cleaning agent. Again, do not use anything abrasive, and to repeat: do not use an eraser!"

The URL for the full article is here:  http://www.learn.usa.canon.com/resources/articles/2012/eos_error_msgs.shtml

As for the focus issue...

In "One Shot" AF mode the camera is also using "Focus Priority" mode... which means it wont take a shot until it can confirm that it was able to lock focus on at least one AF point (if you allow it to choose any AF point it wants) or it could lock focus on your chosen AF point (if you manual selected the AF point.)

In "AI Servo" AF mode, the camera is using "Release Priority" mode... which means the camera WILL take the shot when you fully press the shutter button ... whether it was able to lock focus or not.

Page 85 of your T3 Instruction Manual (not the "Quick" instruction manual... the full version) talks about this problem.

It mentions:

Low light

Low contrast subject (flat colors such as plain walls or blue skies)

Reflections on surfaces or strong backlights

Near and far subjects within the same AF point

Repetitive pattern

Note that when it talks about these, it's referring not to the scene overall... but just the points within the scene where the active auto-focus points are located.

[Skirball]

My bad, xinay on the eraser trick.  Though I find the importance that they put on the gold plating to be amusing.  I wonder if Canon buys Monster Cables too.