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Old 30th of October 2006 (Mon)   #1
curiousgeorge
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Default Focus variations

Have anyone noticed how, when you autofocus on a particular point in the frame, if you repeatedly press the shutter button half way, the focus will jump around ever so slightly backwards and forwards? Even if you keep the camera still.

The lens is therefore unsure as to the distance to focus on and it follows that the image is unlikely to be correclty focused.

This may explain how, when I take two shots of the same image, one is better focused than the other.
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Old 30th of October 2006 (Mon)   #2
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Default Re: Focus variations

Remember, the AF sensor area's actually somewhat larger than the area delimited on your screen. It's quite possible for the AF point you use at any given time to be picking up something outside the area you were expecting. AlsoAF isn't designed to nail the exact same spot every time; it's considered within spec if it's within somthinge like 1/3 the DoF when focussed.
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Old 30th of October 2006 (Mon)   #3
curiousgeorge
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Default Re: Focus variations

It happens when the entire frame is in the same focal plane though, eg the cover of a book.

I wouldn't expect it to vary at all in this situation.
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Old 30th of October 2006 (Mon)   #4
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Default Re: Focus variations

Are you swaying back and forth?
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Old 30th of October 2006 (Mon)   #5
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Default Re: Focus variations

maybe its just rechecking the focus !!!
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Old 30th of October 2006 (Mon)   #6
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Default Re: Focus variations

Quote:
Originally Posted by curiousgeorge View Post
It happens when the entire frame is in the same focal plane though, eg the cover of a book.

I wouldn't expect it to vary at all in this situation.
Read point 2 of my post. Also, your subject needs to be sufficiently high contrast for it to get a good read. The lower the contrast, the more likely you'll have variations. Also the smaller the aperture.
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Old 30th of October 2006 (Mon)   #7
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Default Re: Focus variations

OK, so it's normal behaviour, but in real life shots this happens every time and it's frustrating when you don't know when it has the best focus reading.
I thought someone else would have noticed it.
I would use MF if I could rely on my eyesight.
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