Quote:
A lot of cameras have the motor in the camera itself, the D3100 does not.
In the D3100 the motor is in the lens, which is why that lens will not auto focus....it has no motor in the lens....but can be used in auto focus mode with a camera that has the motor built in.
Sorry I didn't go into detail - trying to multitask! Since I am a visual person, here's a shot of the "screw" in Nikon AF bodies and lenses. The body has a screwdriver type of thing (your's won't have it) and the lens has the screw head. The AF-S lenses do not have this little screw head.