I have to agree with the comment that exposure is not that far off. I think both shots are suffering a lack of punch, or contrast. I played with them on really basic adjustment, and posted. The second shot is also lacking in even lighting between foreground and background (of the guitar to the far end of the neck), in addition to the fact that you are shooting the black tuning machines against a 'black' background. The comment about seperating the guitar from the background will gain you some increase in the 'blackness' of the background, and I think improve the overall shot quality. But, you will still need to pay attention to the light hitting the tuners. Since they are black, the best you can do is make sure they have specular or reflected highlights, to accentuate them from the black background. Don't know if you have modeling lights of any sort, but that would facilitate the fine tuning of lights. I would play with the contrast adjustment (increase) in your menu function. I don't have a D90, so don't know what adjustment that is, but D80 has it, so I'm sure a D90 does as well.