1) if the user only fills in one field the other filed will not be included in the search. the other field will be ignored.
it sounds like you should use the OR seperator for your needs between the 2 keyword search criteria.
2) That is the way it is supposed to work.
the string "cameras" is not contained in the column "Nikon Digital Camera" so nothing is returned.
the includes criteria looks at each word:
cameras = Nikon | NO
cameras = Digital | NO
cameras = Camera | NO
so no result is returned when searching for cameras
a result is returned for camera because that is a word in the column:
camera = Nikon | NO
camera = Digital | NO
camera = Camera | YES