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Old Nov 26, 2010, 3:17 PM   #1
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Default Telephoto focusing problem

I have a Canon Rebel XTI with a Canon 70-300mm lens.
I tried to shoot fighter jets taking off, and the lens would lot focus. It kept "hunting" for focus at the 300 MM position.
I talked to Canon Tech support and they said to send the lens in to tech support, where it is now.
The standard 18-55mm lens focues instantly. I'm assuming the telephoto lens needs repair.
My question is, if the 300mm lens is ok, why did it not focus on the jets?

Anyone else experience this problem?

THANKS!

Last edited by lablover01; Nov 27, 2010 at 8:12 AM.
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Old Nov 26, 2010, 5:34 PM   #2
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Were you using the viewfinder or 'Live View'?

Were you using AI Servo AF?

Had you selected the center AF Point?
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Old Nov 26, 2010, 10:15 PM   #3
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Using the viewfinder, it's all I have on this camera.
AI servo with center point focus.

Last edited by lablover01; Nov 26, 2010 at 10:18 PM.
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Old Nov 27, 2010, 4:36 PM   #4
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i bet it's got something to do with your aperture being f/5.6 when you're at 300mm.
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Old Nov 27, 2010, 4:53 PM   #5
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Quote:
Originally Posted by frank-in-toronto View Post
i bet it's got something to do with your aperture being f/5.6 when you're at 300mm.
That would probably only be a factor in dim light. When shooting jets in daylight, that's probably not an issue.
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Old Nov 27, 2010, 5:03 PM   #6
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It was only a small bet.
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Old Nov 29, 2010, 11:16 AM   #7
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There could be issues with the lens, body or user.
How far away is the subject you speak of? How much of it is filling the frame? Does it focus at the same distance with less zoom? Does it focus on a static target at 300mm in AI servo and single shot?
You also have to keep in mind that the AI servo on an XTi is limited in function with no customization, its on or its off.
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