a function of viewing distance) tends to be steeper, meaning that the binocular system is not able to keep the fixation disparity small, if a target is shifted closer in the range of about 100 to 20 cm. this evidence came from studies using a computer-controlled test stimulus including a central fusion stimulus. all the above measures in studies of eye strain refer to the subjective fixation disparity, because the procedure with dichoptic targets is technically easy and therefore can conveniently be applied in the clinical setting with some commercial test devices. some of the cited studies found, that measures of subjective fixation disparity are a better diagnostic criterion for eye strain than the heterophoria, i.e. the vergence state without a fusion stimulus. the technically more complex eye tracking technology for measuring objective fixation disparity has not yet been investigated in relation to eye strain. remediation of fixation disparity in observers with eye strain given that an observer has a certain fixation disparity and suffers from eye strain, one may consider some of the following ways of remediation. eye glasses with an included prism power is the optical method to reduce a fixation disparity. different procedures have been proposed to determine the required amount of prism for the individual. based on prism-fd curves (fig. 3b), one can find the aligning prism sp0 that nullifies the naturally prevailing fixation disparity sfd0. this test procedure is typically made in near vision of 40 cm, e.g. with the mallett-unit, the disparometer, or the wesson card (see above). experimental evidence for the effectiveness of the aligning prism came from a study of reading speed and corresponding preferences of prism eye glasses. a different approach was suggested by h.-j. haase who proposed a set of dichoptic target tests with both central and more peripheral fusion targets and additional stereo tests that were predominantly used in far vision. such prisms alleviated eye strain and remained stable over time. the usefulness of prism eye glasses has been criticized since the initial fixation disparity may reappear again after some time due to the adaptability of the vergence system. one may consider, however, that vergence tends to be less adaptive in observers with eye strain so that in these observers the prisms may permanently reduce a naturally prevailing fixation disparity. visual ergonomics of a computer workstation may take into account the individual proximity fd-curve: individuals with a larger exo fixation disparity at near may prefer a longer viewing distance where the fixation disparity is smaller. visual vergence training (also referred to as orthoptic exercises or vision therapy) aims to improve the physiological condition of binocular vision with eye movement exercises, including e.g. frequent dynamic vergence changes between near and far vision. the effectiveness has been confirmed both in terms of alleviation of visual symptoms and in better physiological conditions, e.g. the prism-fd curves became more flat. the physiological effect of visual vergence training has also been confirmed for other vergence functions.