using nd filters

The camera should adjust for the filter. HOWEVER if you're using a significantly strong ND filter, it won't be able to adjust for it since it just won't be receiving enough light information.

The nice thing about digital, though, is you can guesstimate the exposure, take the shot, and immediately adjust and reshoot.

One thing I've done is let the camera autofocus and auto-expose without the filter, then throw it into full manual (tweaking the focus if desired/necessary) and adjust the exposure time based on the density of the ND filter. That'll usually get you pretty close right off the bat. For example, if your ND filter drops the light 3 stops, just set your exposure time 3 stops longer - or close the aperture down 3 stops (I thing adjusting exposure time is usually safer, since it won't change the depth of field or introduce possible diffraction).