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| p.1 #2 · Nikon D3100 focusing problem |
My D60 front-focuses with many of my lenses. Ironically, like your D3100 it is also seemingly fine with the 18-55 mm kit lens.
From a search I did, some folks have apparently taken to fine-tuning AF themselves on their non-AF-fine-tune cameras like the D50, D70 and D90, using what look to be the factory adjustment screws found inside the mirror box. For instance see this discussion and this article for some info you might find useful. Not necessarily recommending you do this though, since I have not tried it myself.