The Nikon D90 has a focus motor, the D5100 does not.

The old lenses made for a SLR film cameras and full frame DSLR cameras don't have a focus motor built into the lens. All lenses built for the half frame DSLR cameras have focus motors inside the lens.

Here is a comparison of specifications for the two cameras:

http://snapsort.com/compare/Nikon-D5100-vs-Nikon_D90

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