![]() ![]() Each 0.25-D step of myopic refractive error (outside the depth Once the depth of focus is exceeded, a refractive error degrades visualĪcuity. Of its edges, which is a critical visual acuity task. Detecting a large object requires good differentiation We are approaching an unlit object, we must be able to resolve We are on a strange expressway, we must be able to read the signs. ![]() Objects may be in our path, good visual acuity becomes essential. However, where surroundings are not familiar or where unexpected Or actually seeing low-contrast detail, such as a curb or a raised traffic To navigate by recognition of landmarks rather than by reading signs When navigating in familiar territory under adverse conditions, we tend Thus, reading “acuity” is possible without resolution of the detail. ![]() A street name may be recognized by its length. Q may be more difficult to differentiate because of the similarities Of poor visibility because it has the shape of a triangle. For example, the letter A can often be recognized under conditions We may function by recognition rather than by identification or visualĪcuity. Think we “see” an object when we actually do not because Visual acuity variations also may be perceptual in origin. (Modified from Schwartz JT, Ogle KN: The depth of focus of the eye. Of focus of the eye for a given resolution probability. The width of the curve represents the depth Probability of resolution of a 20/25 (6/7.5) target as a function of theĭioptric blur of the target. ![]() This depth of focus is greatlyĭecreased under conditions of low luminance (large pupils) and reducedįig. Of focus can be expected to be even larger. For higher target luminances and smaller pupil sizes, depth A 50% probability of resolution is maintained to approximately 0.33 D For average pupil sizes, a 20/25 (6/7.5) high-contrast targetĬan be out of focus at least 0.20 D in either the myopic or hyperopicĭirection with no noticeable effect on target resolution ( Fig. In visual acuity with refractive error can be attributed to depth Smallest error as a decrease in visual acuity or contrast, whereas othersĪre happy with 2.00 or 3.00 diopters (D) of blur. REFRACTIVE ERROR, VISUAL ACUITY, AND CONTRASTĬlinical experience demonstrates that patients have a wide range of reactions Materials, and with the development of aspheric lens designs that canīe used for essentially any lens power, lens design has become more important With the increased use of high-index, lightweight lens Of objects from the patient, and the need to maintain normal binocular Surface power or base curve of the lens, taking into account such variablesĪs the vertex distance or fitting distance of the lenses, the distance This involves the proper choice of the front In a spectacle prescription that occur when a patient looks away from Purpose of the spectacle lens design process is to minimize the errors Optical quality and decrease the field of clear vision. Optical effects (aberrations) are present that limit off-axis The patient moves his or her eyes to look away from the lens center, undesirable Spectacle Lens Design Chapter 51BĪ spectacle lens only corrects a patient's refractive error at itsĬenter that is, when the patient is looking along the optic axis. ![]()
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