Hi there. I am new to this forum, looking for some lens advice. I have a 18-105 and a 50mm prime. I take a lot of portraits of my kids and am trying to get really sharp eye focus on my portraits.. I have convinced myself that a sigma 17-70 or a nikon 16 - 85 will get me sharper pics than my 18 - 105. But the more I read then I am starting to think that I could just be spending money for he sake of it as the 18-105 will prob perform just as well as the other 2 lenses. I like the 18-105 as my preference is not to change lenses. Can anyone share their thoughts re this before I waste my hard earned cash that I should not be spending! Thanks