The new Nikon D40 and D40x are the first Nikon bodies without a built in auto focus motor. This means that the any lens used with the D40 and D40x will need the auto focus motor built into the lens. These lenses are called AF-S lenses. Sigma has a large line of compatible AF-S type lenses.
The following Sigma lenses are fully compatible with the Nikon D40 and D40x:
[tags]sigma, lens, lenses, compatible, compatibility, nikon, d40, d40x[/tags]