One thing I do when shooting slabs is back the lighting angle off somewhat, trying to minimize the glare from the slab - even though it may not be affecting the coin itself, it may become something the camera wants to focus on.
Does your camera have the ability to specify focus points? What model Coolpix is it?
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Catman, Gary Burke, Bigg Fredd, coinguybrian, numismo, Johnny54321 - CCF members emeritus, now part of Heaven's Own Coin Club.Our members sell on eBay!