# Confused autofocus on Canon 28-135 USM IS lens



## Sleepy_Sentry (Dec 11, 2009)

I have a refurbished Canon 28-135 USM IS lens I bought from B&H a few months ago. It worked well at first with my Canon Digital Rebel XSi, but lately the autofocus seems "confused."

It can't focus on most objects. It will focus in almost correctly then focus all the way out like i doesn't know what to do. Then if I keep on pressing the shutter halfway, it will try to focus again but fail. The problem occurs with IS on and off. I have to use manual focus to take a shot. 

I tried the lens on an older Rebel 2000 film SLR and it worked fine with no autofocus issues whatsoever. I tried another lens on my Rebel XSi and the autofocus on that functioned perfectly as well. So it's only a problem with the 28-135 lens on the XSi body.

Does anyone know what the problem could be? The problem itself began during the lens' three month warranty, but I didn't have time to diagnose it until now because of my busy college schedule. Could it still be covered under warranty because of when it began?


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## NateWagner (Dec 11, 2009)

does this happen from all distances and in all lighting conditions?


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## Sleepy_Sentry (Dec 11, 2009)

NateWagner said:


> does this happen from all distances and in all lighting conditions?


Yes, but it seems to be worst in dark conditions. I have never tried focusing on anything more than 30 feet away so it might work on something really far away.


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## Dao (Dec 11, 2009)

If you use the AF assist lamp (build-in flash), will that help?


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## Sleepy_Sentry (Dec 13, 2009)

Thanks for the advice Dao. Before trying that, I actually switched the AF mode to single-sensor and it fixed the problem. For some reason it was in AI Servo.


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## NateWagner (Dec 13, 2009)

yeah, AI Servo would do it.


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## Dao (Dec 13, 2009)

lol ... no wonder.  Glad you figured that out!


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## Arsi (May 24, 2011)

Sleepy_Sentry, In May 2007 you wrote: 





Sleepy_Sentry said:


> "  I actually switched the AF mode to single-sensor and it fixed the problem. For some reason it was in AI Servo."


I have a Cannon D40 with a Canon 28-135 lens and it stutters when trying to focus in the Automatic mode with the AF switch in ON position. The shutter does not actually get triggerred and does not take a picture. All manual modes work fine. I am hoping your solution may work for me as well. But I am not sure how to switch the AF mode to single sensor and I don't see where (and if) it is set to AI Servo on my camera. What I have is an AF|MF position switch above the Stabilizer ON|OFF switch.
Any help that will prevent me from having to buy a new lens is appreciated


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## Bitter Jeweler (May 24, 2011)

Do you have the manual for your camera?


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## KmH (May 24, 2011)

Is a Canon lens compatible with a Cannon camera? I've never heard of a Cannon D40, but I know *Nikon* used to make a *D40*, and Canon used to make a 40D.

There are some shooting situations where auto focus just doesn't work very well:

When there is little or no contrast between the subject and the background.
When the chosen focus point has objects at diffferent distances from the camera
When the subject is dominated by regular geometric patterns
When the focus point contains areas of sharply contrasting brightness
When the subject conatins many fine details.
Often times the issue is the photographer is using an inappropriate focus mode or focus area mode and/or there is just not enough light for auto focus to work..


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## Arsi (May 29, 2011)

Not the original manual that came with the camera. I had a copy on my computer that I cannot find anymore. I have however found bits and pieces of video on YouTube that describe various functions.
I found where in the display it says that it is set at AI Focus. When I press the AF-Drive button and simultaneously turn the wheel, it does not change to other AF modes. I will keep trying! Thanks for the suggestion to go back to the basics!


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## Arsi (May 29, 2011)

My apologies. Obviously I am a newbie. I should have said EOS 40D. The lens came with the camera.
The lens has the same behavior on another 40D body. After paying more attention, I now see the autofocus mode in the display set at AI Focus. I follow the suggestion in a previous post that said: 'I actually switched the AF mode to single-sensor and it fixed the problem. For some reason it was in AI Servo." and change it to single sensor to see if it helps. I got hold of YouTube video that show how to do that although my camera does not "yet" behave the way it is shown in the video. I will keep trying.


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## KmH (May 29, 2011)

Arsi said:


> Not the original manual that came with the camera. I had a copy on my computer that I cannot find anymore. I have however found bits and pieces of video on YouTube that describe various functions.
> I found where in the display it says that it is set at AI Focus. When I press the AF-Drive button and simultaneously turn the wheel, it does not change to other AF modes. I will keep trying! Thanks for the suggestion to go back to the basics!


You can download a PDF of the 40D User's manual any time you want from www.canaonusa.com: http://gdlp01.c-wss.com/gds/6/0900008236/01/EOS40D_HG_EN.pdf


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