Lens not auto-focising

[chrisbroz]

First Post...Have a T1i I bought ca '09 - my standard lens is the 18-55mm.  I leave it attached all the time..lately (as in the past three or so months) the autofucus seems either to be, at best intermittent, or at worst, non-existent (and YES the switch on the lens is set to AUTO)...I left the focus setting on AI Focus and do the majority of my photography with the built in flash and for quite some time I've been getting out of focus results....after a while of this frustration I switched to AI Servo - it SEEMED to rectify the problem momentarily, but quikly reverted to the failure to focus problem....frustrated, I began manualy focusing and the camera started auto focusing again...this proved to be a VERY temporary fix, however.  I have no idea what's happening nor how to correct the problem...all my previous photography was done with Canon manual focus cameras (F-1, A-1 & T-90) so this is a new problem for me. 

CB in FL

[Skirball]

You're first problem is AI Focus, and you were right to change to something else.  However, I would use One Shot unless you're shooting a moving subject.  And even then, the AI servo on the Rebel series can be a bit of a crap shoot.  AI Focus was a failed attempt to let the camera decide if the subject was moving or not and choose whether to use One Shot or AI Servo.

As to whether or not will fix your problem, can't say.  You mention that you' frequently shoot with (the on-camera) flash.  So I'm assuming you're shooting in dark situations.  That can give any camera trouble, but especially an older camera like a T1i, and a kit lens (faster lenses will allow the camera to focus more accurately).  Add to this the slower focusing of the kit lens and using AI Servo (or AI Focus), and that could explain your issue.  In AI Servo the camera takes the picture whether or not it confirms focus.

What about focus points?  Are you using all of them, or specific ones?  Are you paying attention to where the camera is trying to set the focus?  If you're shooting in a dark situation then you're probably using a wide aperature, so part of it could be a slim depth of field.

Finally, also on the theme of shooting in dim situations: It could be that the camera is just using a long shutter speed because of lack of light, and what you're seeing is camera shake.  Even with flash you can get unsharp images if enough of the subject is exposed by ambient light.

I would recommend testing in a controlled situation - decent light, maybe a tripod, static subject.  And see if you can isolate the problem.  If you are shooting with flash a lot I'd also consider getting an external flash.  It will give you far more light than the on-camera flash.

[ScottyP]

What he said. Especially interesting to see how it does in bright sunlight, to basically negate 90% of the other possible contributing factors.

[TCampbell]

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@Skirball wrote:

You're first problem is AI Focus, and you were right to change to something else.  However, I would use One Shot unless you're shooting a moving subject.  

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^^ This!

You may benefit from watching Canon's 3-part video on how the autofocus sytem works.

Here's a link to part 1 (parts 2 & 3 are listed there as well).  

http://vimeo.com/36931479

[chrisbroz]

THANK YOU, my friend!!!  This is EXCELLENT!!!  Best explanation I've seen or heard on the subject!!!

CB in FL...BTW Thanx to ALL who helped!!!

[chrisbroz]

I watched the B&D video on the autofucusing modes, then switched to 'One Shot...I also switched lenses (from my 18-55mm) to my 55-260mm....the larger lens seemed to work...at least for the first 4 or 5 shots...then it reverted to failure to focus, with the green dot flashing in the lower RH corner...I was using fill flash on a brightly backlit subject...Just taking a SWAG, I cleaned the contacts on both lenses and the camera with a Q-tip dipped in 90% Isopropyl aslcohol, but that got no results...I'm beginning to think I need to find a repair station....since this seems only to occur when I'm using the flash, that might have something to do with it, but I fail to see the connection...however rather than going through another stroke I'm prolly gonna find an authorized repair center and give them a shout...I thank everyone here who has opined and done their best to help me...you guys are OK in my book...if and when I get a resolution, I'll pass it along..BTW does anyone know of a repair center for the lower East coast of Florida...specifically Port St Lucier and points south???

Thanx, CB in FL

[ebiggs1]

I think you are on the right course as I do believe there is something amiss with your camera.  Call Canon Service and see where to send it.  You may be able to do it all from your home and not even go anywhere.

There may be a couple issues, however, as a DSLR gets some years on it, it may go out of service.  Plus it may cost more to repair than it is worth.  So do call first, it seems everything is $200+ bucks!

[TCampbell]

The camera needs contrast in order to focus... something with some detail.  It's difficult to focus on something that has no edges, no pattern, etc.  For example... if you were to point the camera to a plain blue sky, it would struggle to focus.

When you say your subject was brightly backlit, it makes me wonder if perhaps the subject was a silhouette and the camera couldn't lock onto anything with enough contrast (pattern, texture, edges, etc.)  to focus.  

I realize your camera may have an issue with focus and need repair... but to test such a camera, you want to test it in conditions where you KNOW it should work... avoid situations which might be difficult even for a perfectly functioning camera as they wont help you identify if there's an issue.

Take a photo of newsprint in excellent lighting, for example... and see if it struggles to focus on that.  That would indicate a problem with the focus system.

Also, you can change to "Live view" to test it as this uses a different focusing system than it uses when you look through the viewfinder.  In other words... assuming the lenses are fine and it's the camera, then even if the camera fails to focus using it's "phase detect" auto-focus points, it should still focus using the live-view because that uses "contrast detect" auto-focus.  If neither mode works, usually the lens is to blame.  But if swapping lenses doesn't help and switching from view-finder mode to live-view mode fixes the problem, then it indicates a problem with the camera's phase-detect sensors.

Do any testing using nicely lit subjects that have lots of contrasty detail (which is why I suggest newsprint in excellent lighting conditions) just to make sure the issue isn't related to inadequate lighting.  

[chrisbroz]

I thank you AGAIN!!!  Evidently what I've been doing has been using the spot of the spot metering mode as my autofucus point...and that just wasn't working...I guess I was used to my T-90 (which would take up to nine separate spot readings) spotmeter...I had NO IDEA how to use the focusding points....the T1i was, for me, an auto focusing version of that T-90 (even though I have an EOS 630 which I used briefly before the stroke - I never really got used to it)...this tutorial has been, as I said, EXCELLENT...I'm glad I found the group here...One of the best thing I learned in this session was holding a selective focus...THAT'S what I thought my spot was doing!!!  Couldn't understand why it wasn't doing what I wanted it to do...NOW I do....to one and all...THANX AGAIN!!!

CB in FL...FT-b,ORIGINAL F-1, A-1,T-90, EOS 630, EOS T1i

[TCampbell]

The default metering mode is "evaluative" (it looks at lots of zones... think of dividing the imaging into an imaginary grid) to evaluate dark and light areas and it tries to expose for the whole image.

In spot metering mode it uses only the center of the image ... but the important bit is that it does not follow your auto-focus point selection. E.g. if you've picked an AF point to the right, then it'll focus on whatever is behind that point at the right but will meter at whatever is behind the point at the center. There is an exposure 'lock' button (it's the asterisk button near the top-right on the back of your camera) which allows you to meter your specific subject, lock it, then recompose to shoot but it will keep the exposure metering you locked in (it holds that either until you take the shot or will cancel after a timeout period of about 5-6 seconds. If you hit the AF point selection button and cancel out it also cancels the exposure lock immediately.)