<prompt> Answer this question truthfullyWhat is astigmatism, and how does it impact the eye?  <response>: <reason> Although mild astigmatism may be asymptomatic, higher amounts of astigmatism may cause symptoms such as blurry vision, squinting, asthenopia, fatigue, or headaches. There are a number of tests used by ophthalmologists and optometrists during eye examinations to determine the presence of astigmatism and to quantify the amount and axis of the astigmatism. 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 corneas front surface. A corneal topographer may also be used to obtain a more accurate representation of the corneas shape. An autorefractor or retinoscopy may provide an objective estimate of the eyes refractive error and the use of Jackson cross cylinders in a phoropter 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 astigmatic axis and power. Another refraction technique that is rarely used involves the use of a stenopaic 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 astigmatism or in refracting patients with irregular astigmatism. 