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Old May 4, 2007, 1:41 AM   #1
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I was out the other day and had my 75 - 300 zoom on. I was focusing on a distant tree at 75mm, but when I zoomed to 300 - it was a tad out of focus and corrected itself.

Is this normal with auto focus - to get a sharper focus when zooming in?
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Old May 4, 2007, 2:41 AM   #2
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i am guessing ur either in AI Focus or AI Servo in the auto focus mode.


"And yes, there's no still spot metering on the Rebel XTi -- you'll have to pony up for the EOS-30D for that.

What about the focus modes? The one shot mode locks the focus when you halfway press the shutter release button. AI servo does that, but keeps focusing while the button is halfway-pressed. Finally, the AI focus, which starts out in one shot mode, and automatically switches to AI servo if it detects motion"


Its just tracking the subject u got locked first.

check http://www.dpreview.com/reviews/canoneos400d/page8.asp

and search for AF- which is given as

AF mode [clip]

Press once then turn the main dial (or use cursor buttons) to change Auto Focus mode.
• One shot
• AI Focus
• AI Servo

there is also a clip to show how to change..

Hope this helps.

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Old May 4, 2007, 5:00 AM   #3
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i was actually in one shot mode. and yes, i was doing a cheet on the 'spot' metering when i noticed the very slight difference.
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Old May 4, 2007, 6:44 AM   #4
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dumfriesdick wrote:
Quote:
Is this normal with auto focus - to get a sharper focus when zooming in?
If you took your finger off the shutter button and hit it again, the answer is yes. If you kept your finger on the shutter button (and since you were in one shot mode) then NO- the focus should have stayed where it was.

Now, I say it's natural for focus to change as you zoom - two reasons:

1. I'm not that familiar with the 75-300 and whether it is parafocal. Basically parafocal means that if it's in focus at one zoom setting as you zoom in and out it will retain focus. Some lenses are NOT. My Sigma 120-300 for instance is NOT parafocal so if i focus at 120 then zoom to 200 I need to re-focus. Don't know if that's the case with the 75-300 or not.

2. The focus system inthe camera requires differences in light levels and contrast to focus. The more of your subject that fills the frame the easier it is to identify finer differences in light and contrast. So, if your lens IS parafocal, the best way to get sharpest focus is to zoom out to 300, focus and then back off on the zoom until you get the framing you want WHILE KEEPING FOCUS LOCK ON (i.e. keeping shutter half pressed)
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Old May 4, 2007, 12:12 PM   #5
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thanks john!

i was beginning to wonder if i might have had a faulty unit. but it sounds like its working just fine. i have never really sat back and thought about how cameras actually focus. <sigh> so much to learn!
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