A number of tests are used during eye examinations to determine the presence of<unk>  and to quantify its amount and axis. A Snellen chart or other eye charts may initially reveal reduced visual acuity. A keratometer may be used to measure the curvature of the steepest and flattest meridians in the cornea's front surface. Corneal topography may also be used to obtain a more accurate representation of the cornea's shape. An autorefractor or retinoscopy may provide an objective estimate of the eye's refractive error and the use of Jackson cross cylinders in a phoropter or trial frame may be used to subjectively refine those measurements. An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to determine the<unk> igmatic axis and power. A keratometer may also be used to estimate<unk>  by finding the difference in power between the two primary meridians of the cornea. Javal's rule can then be used to compute the estimate of<unk> . A method of<unk>  analysis by Alpins may be used to determine both how much surgical change of the cornea is needed and after surgery to determine how close treatment was to the goal. Another rarely used refraction technique involves the use of a stenopaeic slit (a thin slit aperture) where the refraction is determined in specific meridians – this technique is particularly useful in cases where the patient has a high degree of<unk>  or in refracting patients with irregular<unk> . Classification There are three primary types of<unk> : myopic<unk> , hyperopic<unk> , and mixed<unk> . Cases can be classified further, such as regular or irregular and lenticular or corneal.
Astigmatism
Paragrammatism
Apophlegmatism
Agrammatism
Social-emotional agnosia
Boanthropy
Integrative agnosia
Visuospatial dysgnosia