Abstract: Sheet feeder having adaptive mechanism for assuring dependable automatic operation when the stack of sheets to be fed consists of sheets having varying thicknesses and weights. In a sheet feeder having pickers to move a document from the top of a stack of sheets supported by a moveable table to a feed-in position, the latter possibly including a vacuum belt for moving the document, sensors supply signals to indicate a sheet has been picked. After moving the pickers and table to initial positions, the controller moves the picker to a predetermined reference position. If a sheet is picked above or below the predetermined initial position, the position of the table is adjusted accordingly.