6533b7d5fe1ef96bd12653ab

RESEARCH PRODUCT

Depth-of-Field of the Accommodating Eye

subject

description

Our eyes project information from a three-dimensional world to a basically two-dimensional surface, corresponding to the photoreceptor plane in the retina. In theory, only one plane or surface of world can be in focus at one time. However, the eye exhibits a certain tolerance to out-of-focus images, a feature that is known as depth-of-focus (DOF). The corresponding distance range in which the objects are seen “clearly” is known as depth-of-field (DOFi). Although DOF and DOFi refer to an interval of distances or a dioptric range in the image and object space, respectively, both parameters define a similar concept and are usually interchangeable. This article will mainly refer to DOFi because it represents a more practical parameter, which can usually be determined through direct measurement. It is a well-known fact that certain optical and neural factors can change the DOFi. One of the most important ones is pupil size1; the presence of chromatic2,3 and monochromatic aberrations4–6 play a significant role as well. The precise value of DOFi also depends on the specific way DOFi is defined. Whereas some definitions are related to an absolute minimum value of visual acuity (VA),2,7 there are others based on relative VA values, for example, dioptric range over which VA exceeds some percentage of its optimum value.8–10 In practice, DOFi is usually based on a simple letter legibility criterion. In this case, the letter size plays an essential role in the final DOFi value.11,12 As a simple example, most presbyopes recognize the visual loss at near distances when trying to read small text, but not with large font sizes, which are nonetheless out-of-focus by the same amount. Depth-of-field of the presbyopic eye is a critical determinant of the prescribed add power or the design of contact or intraocular lenses13,14 and refractive surgery.15 It has been proposed that DOFi is responsible for some or most of the accommodative errors (lead or lag).5 Although there are some studies that have examined the origins of accommodative errors,10–12 the true origin of the accommodation error remains an unanswered question. Moreover, it has been shown quite recently that the error of accommodation can actually be a “false accommodation error” resulting from criterion differences between the subject (for subjective measurements) and the measuring device (for objective ones) when choosing the best image.16,17 There have been numerous studies devoted to measuring the DOFi on either side of the far point (FP) of paralyzed lens or presbyopic eyes.1,5,10,18 However, there is a lack of data regarding the DOFi as a function of the accommodative state. The reason behind this lack of data is the difficulty to accurately measure the DOFi of the accommodated eye because accommodation cannot be blocked—neither voluntarily nor involuntarily (by means of drugs) to remain at a specific desired accommodation level. The DOFi, although accommodating, can certainly differ from that of the relaxed eye because the pupil size and the aberration pattern change with accommodation.16,19–23 Several authors have used an optical system in which the subject focuses his or her attention on a stimulus located at a given distance, while another stimulus close to it can be moved voluntarily by the subject until its image is no longer clear and sharp, that is, until it falls out of their DOFi.2 However, it is quite difficult to perform this task with accuracy, because VA decreases very rapidly with stimulus eccentricity.24 As far as we are aware, there are only three studies that measured DOFi for objects located between the far and near points.2,25,26 Campbell2 used a stimulus made up of a vertical row of three dark discs. The vergence was set during the experiment to a fixed −2 diopter (D) value and it was used at a fixation point that presumably led to a steady accommodation state. Two plates, one with a single disc on the left of the vertical row and another one having the disc on the right side, were moved by the subject along an optical bench so as to find the distance range in which both discs could be seen clear simultaneously. In 1998, Mordi and Ciuffreda25 used a similar technique to study a large population having a wider age range. For each subject, they measured the subjective DOFi in the middle of the linear part of the stimulus-response curve. Moreover, in 2010, Yao et al.26 performed objective and subjective measures of DOFi for 0, −1.5, and −4 D stimulus vergence under the hypothesis that the accommodative and perceptual systems have different abilities to detect blur. The system used was similar to that from Campbell, but only measured the distal half of the DOFi. Therefore, estimating the total DOFi with this approach assumes that DOFi is symmetric in the object space.5 Although accommodation leads and lags may be influenced by DOFi, there are few studies of DOFi with varying levels of accommodation, and therefore, none of the previous studies answer the key question: why is the actual accommodative response different from the ideal one for most stimulus vergences? In this work, we propose a novel methodology based on wavefront technology to provide an answer for this question.