The FXScript Reference

The duration value returned by GetLimits is whole frames, not fields, regardless of whether the script calling the function uses the fullframe tag or not. This also means that the returned image data is full height, and not field-height.

When calling frame data for field-based effects, some sort of field-extraction routine should be used to guarantee that the desired field is returned.

This is one of the most confusing aspects of working with video data from other points in time.