vision In casual conversation, the term blur can be used to describe any reduction in vision. However, in a clinical setting blurry vision means the subjective experience or perception of optical defocus within the eye, called refractive error. Blur may appear differently depending on the amount and type of refractive error. The following are some examples of blurred images that may result from refractive errors:The extent of blurry vision can be assessed by measuring visual acuity with an eye chart. Blurry vision is often corrected by focusing light on the retina with corrective lenses. These corrections sometimes have unwanted effects including magnification or reduction, distortion, color fringes, and altered depth perception. During an eye exam, the patient's acuity is measured without correction, with their current correction, and after refraction. This allows the optometrist or ophthalmologist ("eye doctor") to determine the extent refractive errors play in limiting the quality of the patient's vision. A Snellen acuity of 6/6 or 20/20, or as decimal value 1.0, is considered to be sharp vision for an average human (young adults may have nearly twice that value). Best-corrected acuity lower than that is an indication that there is another limitation to vision beyond the correction of refractive error. The blur disk Optical defocus can result from incorrect corrective lenses or insufficient accommodation, as, e.g., in presbyopia from the aging eye. As said above, light rays from a point source are then not focused to a single point on the retina but are distributed in a little disk of light, called the blur disk. Its size depends on pupil size and amount of defocus, and is calculated by the equation (d = diameter in degrees visual angle, p = pupil size in mm, D = defocus in diopters). In linear systems theory, the point image (i.e. the blur disk) is referred to as the point spread function (PSF). The retinal image is given by the convolution of the in-focus image with the PSF. See also Bokeh Shape from defocus References Smith, Warren J., Modern Optical Engineering, McGraw–Hill, 2000, Chapter 11, Optics Geometrical optics