While the Personal Vision Tracker can measure the refractive error of your eyes, there is a limitation to what range it can measure. Users who have a spherical measurement of +8.00D to -10.00D are within the limitations of the PVT. Those users with current prescriptions outside of that range may not receive a proper reading of their hyperopia or myopia. The measurements are accurate within +/- 0.5D. Those users who also have astigmatism, with a cylinder measurement of up to -4.00D are also within the limitations of the PVT. Those with a cylinder above -4.00 diopters may not receive a proper reading of their eyes as well. The axis readings of a user's eyes are not affected.