Lens for my Nikon D3100?

Just need some help about choosing a lens for my d3100. I basically want to take photos of people and sharp photos.probably a max aperture of 2.8 or maybe lower but I want a sharp one. Also I want a zoom as well.better than an 18-55 that I'm using at the moment. I want a lens with zoom and a sharp photo result I want to bring more the subject into focus.

Nikon 18-105mm. Max aperture: 3.5
sigma 28-105mm.max aperture: 2.8
nikon 28-70mm. Max aperture: 2.8
check out this website: www.kenrockwell.com/nikon/nikkor.htm

Try using a tripod and the timer first without VR using apertures from f/5 up (larger f/numbers) and see if your pictures get sharper.
If you want the background blurred, simply position the subject as far away as possible from it (the background) and use a larger aperture size (lowest f/numbers).

Good portrait lens - Nikkor 50mm f1.8. Won't autofocus on your camera body though, need to manually focus it.
Good zoom - Nikkor 70-300mm, will give you the reach you want, but not the sharpest. If you want quality optics in a telephoto, you going to have to shell out a lot of cash (probably about twice what you paid for the camera body).

I would consider Nikon 70-300mm f/4.5-5.6G ED IF AF-S VR Nikkor Zoom Lens for Nikon Digital SLR Cameras
70-300mm telephoto zoom lens with f/4.5-5.6 maximum aperture for Nikon digital SLR cameras
Vibration Reduction (VRII) minimizes effects of camera shake to produce sharper images
2 Extra-Low Dispersion (ED) glass elements delivers super contrast and resolution performance
Internal Focus (IF) system provides fast and quiet autofocusing; 4.9-feet close focus range
Measures 3.1 inches in diameter and 5.6 inches long; weighs 26.3 ounces; 5-year warranty
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