To prove that switching from preserved to unpreserved antiglaucoma medication improves the signs and symptom of OSD is, however, not easy. This is on the one hand related to the physiological variability of these parameters over time and on the other hand to the poor association between signs and symptoms. Recently, a new method for measuring tear film thickness (TFT) using ultra-high resolution optical coherence tomography (OCT) has been introduced. Using this method, it has been shown that there is a correlation between reduced TFT and OSD symptoms. In the present study, it is hypothesized that changing patients who are on preserved prostaglandin formulations to preservative free tafluprost may be associated with an increase in TFT.