Caution ! Dynamic AF versus single point AF... !

I would suggest that you aren't testing it properly.  You should be able to press and hold AF-On in the blue sky and let it hunt and fail to focus. At that point, it should stop attempting to focus. Do NOT release the AF-On button.  Continue holding the AF-On button in while slowly moving the selected AF point toward a high contrast target.  As soon as one of the 21 AF points sees the new target, it should immediately lock on the new target.

It is best if you use a precise AF target, such as a large, straight, solid black line on a very large, white background.

Kerry

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my gallery of so-so photos
http://www.pbase.com/kerrypierce/root