I just thought I'll share an interesting analysis (actually its more of an experimentation rather than analysis) with all of you. Recently whenever I would sit down to start working on my pictures I would notice that the area I wanted to be tack sharp (yeah I'm a sharp junkie) wouldn't really turn out that way. I know from experience that there are a lot of factors that could contribute to this - some of them would be lesser shutter speeds causing motion blur, higher ISO noise, a camera focus issue, a lens issue (Somewhere in my mind I was thinking this may be the problem - especially when I had dropped my 18-70mm lens on the floor last month) and of-course a factor typically overlooked in some of these cases is the sweet spot of the lenses - i.e. what is the aperture at which the lens shoots the sharpest (majority of the lenses produce very good results when stopped down a little bit and the kit lenses typically would produce really sharp images at an f/7.1 to f/9.0 range.
In order to get to the bottom of this, I had to carry out a few experiments and eliminate each cause one by one. So I started out with changing my lens (as I mentioned earlier I was thinking that my lens may be causing this - although the Nikon kit lenses are pretty well built, but still not so well as the more professional ones). I observed that the issue would continue to creep in even in my other lenses - so I knew now that its not a lens issue. I then started shooting with both my test lenses at f/8.0 and the problem would still persist. I also clicked a few pictures at higher shutter speeds (1/120th of a second or so) to eliminate any motion blur. None of these yielded the result I was looking for, although in the process I had eliminated all possibilities contributing to this irritating phenomenon except for the camera itself. With my attention now focused on the camera I started with the most obvious place to start - I went into the custom settings menu options and started looking at my focus options. I reset auto focus to single area (I waned to make sure I choose the focus area in my picture instead of letting the camera choose the focus area), followed by 3D focus tracking - no results. The one item that I reset and which nailed the problem for me was the focus area - I noticed that the focus area was set to wide - I believe I had set it myself; I set that back to normal - took a few pictures and bang I had nailed it!
For clarity here are the steps I followed to resolve the problem:
Menu options > Custom Settings Menu > AF Area Mode - changed to Single Area - now I choose the focus are point manually while shooting - gives me greater control on where I want to focus in my composition
Menu options > Custom Settings Menu > Center focus area - changed to Normal zone - this had been set to wide zone
Apart from this I still believe developing the right technique is very important to nail the overall quality of images (I'll keep saying though that composition and lighting are the prime factors to get a good picture - sharpness won't take your pictures to the next step - it really is the next step once you get a good picture) - shoot at higher shutter speeds (even if you have to use a high ISO especially now that cameras have really evolved in the high ISO noise space, moreover you can fix noise satisfactorily but not blur), shoot while stopping down a bit, if you do have to shoot with a shallow depth of field - make sure you look through your view-finder (or if you have a live view option in your camera use that), make sure you're focusing accurately and try manual focus wherever possible. Happy shooting!