EOS Rebel XSi won't consistently take pictures

[schooley77]

Seems to take pictures in MF, but not always in AF.  Sometimes the flash starts quickly flashing and acting like it's trying to focus, but never snaps the picture.  But it's not all the time.  Seems to work better when there is lots of light- but not so well in low light with AF.  Any ideas? Thanks!

[Peter]

AF sensors need contrast to autofocus. If there is no contrast on the objekt the autofocus will continue to search. The flash pops up to try to help in dark places.

It is also possible that you have gotten dust on your AF sensor. Easy to remove with this.

Anyway. Go to M, Av, Tv or P and try to use the centre point at an object with high contrast.

[ScottyP]

That describes exactly how cameras behave when trying to focus in less light than they are designed to focus in. My old T3i drove me crazy with that, and the micro flashing focus assist, and buzzing during the pause was awkward when trying to shoot people. The camera will do this in situations that are not really dim to your eyes.

A full frame camera or even a higher end crop camera will do a better job with this. A 6d will autofocus on center AF point at -3 EV, and the 5d3 will AF at -2EV. When you upgrade/replace the camera some day look at that spec.

As stated above it helps to put the center AF point over some kind of high contrast feature like a brighter object against the dim background, a hard edge, a bold pattern, etc.
Some speed lites will also help with more AF assist light.

[RobertTheFat]

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@ScottyP wrote:
That describes exactly how cameras behave when trying to focus in less light than they are designed to focus in. My old T3i drove me crazy with that, and the micro flashing focus assist, and buzzing during the pause was awkward when trying to shoot people. The camera will do this in situations that are not really dim to your eyes. ...

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Scott touches on an important point that many photographic newbies may not fully grasp: the dynamic range of the human eye is far greater than that of any camera, Moreover, the eye operates in "Tv", not "Av", mode, opening up or stopping down as needed. So it's almost impossible to second-guess a camera's metering system. You just don't see the world the way it does.