The Uniocular Field of Fixation (UFF) refers to the maximum uniocular excursion of the eyes in various directions of gaze.  This is assessed by presenting the patient with an optotype of a size just above the foveal acuity threshold.  The optotype is moved from the centre of the field outwards until the patient reports that it "goes blurred".  This occurs at the limit of excursion in that direction of gaze and the target moves off the fovea.