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However, they darken the image and the grid blocks some of the finer details, requiring slight movements of the camera for the whole chart to be somewhat comfortably read. Pinhole glasses , also known as stenopeic glasses , are eyeglasses with a series of pinhole-sized perforations filling an opaque sheet of plastic in place of each lens.
The image at the sagittal focus is a short line, oriented in the direction of the tangential plane; images of spokes radiating from the center are sharp at this focus. In between these two foci, a round but "blurry" image is formed. This is called the medial focus or circle of least confusion. This plane often represents the best compromise ...
Optical aberrations of the eye that decrease visual acuity are at a maximum when the pupil is largest (about 8 mm), which occurs in low-light conditions. When the pupil is small (1–2 mm), image sharpness may be limited by diffraction of light by the pupil (see diffraction limit). Between these extremes is the pupil diameter that is generally ...
A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862 as a measurement tool for the acuity formula developed by his professor Franciscus Cornelius Donders .
A number of tests are used during eye examinations to determine the presence of astigmatism 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. [23]
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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.
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