Autofocus failure

[bryan5b]

I have a Rebel XSi with 18-55 mm lens.

The autofocus seems to be broken. When I try to shoot, the AF point lights up and the focus confirmation light blinks, indicating that focus cannot be achieved. Most of the time the focus ring does not turn when I do this. I have found that if I switch to manual focus and turn the focus ring all the way to the right to focus up close, then switch to autofocus, the camera does turn the focus ring on the first try, but then gets stuck. 

The problem is somewhat intermittent. It would come and go for a while. Right now it is always failing.

Also if I focus manually and then switch to autofocus and shoot at the same distance, the camera does shoot.

I tried swapping the lens with another camera. The problem seems to be with the camera body rather than the lens. 

I tried charging the battery, resetting the camera settings, and updating the firmware. No luck.

Any suggestions?

Thank you very much!!

Bryan

[Waddizzle · Accepted solution]

The usual suspect reasons for no auto-focus are: not having enough light; not having enought contrast; being too close to the subject; using an extension tube, which causes the f/stop value to be too high. 

The kit 18-55mm lens has been known to have AF issues when people accidentally turn the focus ring, instead of the zoom ring, when the AF switch is in the AF position.  The lens lacks AF manual override, so turning the ring in AF mode can jam up the focusing motors.  Usually, people are reporting that the lens rattles, when shaken, if jammed focus motors is the case.

Do you know how AF systems work?  Specifically, the light paths that are created wtihin the camera.

http://media.digitalcameraworld.com/wp-content/uploads/sites/123/2013/11/Phase_detection_autofocus_c...

Digital electronics do not have a tendency to fail intermittently, just analog circuitry does that.  It is possible that your camera just may simply need a cleaning.  The AF electronics are at the bottom of the camera, as shown in the illustration at the link that I posted.

[bryan5b]

Thank you very much for the suggestion.

It is helpful for me to see how the autofocus system works. I bet the problem does have to do with the autofocus circuitry inside the camera. The question is,what do I do about this!?  Anybody ever opened up the camera body?

Regarding cleaning - I did a search on "clean dslr camera". I read that I should blow dust off and wipe clean the lens and blow dust off the image sensor. I can look inside the camera and I don't really see dust in there.

Do you suggest anything else in particular regarding cleaning?

Also, I did earlier try wiping the electrical contacts of the lens and the camera with alcohal swabs. It didn't help.

Thanks, 

Bryan

[ebiggs1]

"I did earlier try wiping the electrical contacts of the lens and the camera with alcohal swabs. It didn't help."

It rarely does.  I, personally, have never seen it work!  It is not any of the other 'cleaning' issues.  The point to ask, did the lens work or not on the other camera.  That will provide your answer.

[ebiggs1]

"I tried swapping the lens with another camera."

What happened when you tried the lens on a different camera?  If the lens worked as expected, then you answered your own question.  Didn't you?  The lens is fine.

But if the lens didn't work or acted the same, the lens is faulty.  Simple.

[bryan5b]

ebiggs1 - When I tried the lens on another camera, the lens seems to work fine. So I think the problem is with the camera body, not the lens.

Thanks!

[ebiggs1]

" I tried the lens on another camera, the lens seems to work fine."

The lens doesn't AF.  The camera does.  The lens has no idea whether it is in or out of focus.