Examples of refractive error are myopia, hyperopia, presbyopia and astigmatism. Types of ametropia include myopia, hyperopia and astigmatism. Other types of refractive error are hyperopia, astigmatism, and presbyopia. Low order aberrations include Myopia (positive defocus), hyperopia (negative defocus), and regular astigmatism. Myopia (short sightedness) requires a divergent lens, whereas hyperopia (far sightedness) requires convergent lens. This was mainly due to opacities in both corneas, complicated by hyperopia and astigmatism. Patients with the incomplete form can present with either myopia or hyperopia. The PERK study demonstrated that people who undergo RK continue to drift toward hyperopia ("farsightedness"). This procedure can correct large amounts of myopia and hyperopia. Sheep eyes exhibit very low hyperopia and little astigmatism. The opposite of "myopia" in English is "hyperopia" (long-sightedness). Only about 20% of children with hyperopia greater than +3.5 diopters develop strabismus. It causes a loss of accommodation, far sightedness (hyperopia), and a deep anterior chamber. Other signs may include hyperopia due to accommodative paresis, photophobia and difficulty reading. It is also used to evaluate accommodative ability of the eye and detect latent hyperopia.