(C) Control system model of accommodation. This theory of lags and leads states that the eye, when presented a polychromatic stimulus, strategically focuses the longer wavelengths in that stimulus when it is far (left side of graph), middle wavelengths when it is at medium distance (middle), and short wavelengths when the stimulus is near (right). A lead is schematized on the left where the stimulus is far and the eye has focused nearer than that. An accommodative lag is schematized on the right where the stimulus (black line) is near and the eye has focused farther than the stimulus. It exhibits errors relative to the ideal response: lags at large diopter values (near distances) and leads at small diopter values (far distances). The blue curve represents commonly reported data.

The gray diagonal line is where accommodative response would precisely match the accommodative stimulus. For reference, the distances in meters are shown on top. Accommodative response in diopters is plotted against stimulus distance in diopters. (A) Accommodative stimulus–response curve. Stimulus–response curve, chromatic aberration, accommodation control system, and visual acuity.

Our purpose here is to investigate whether accommodative errors-lags and leads-are as large as commonly thought. Stimulus–response curves like the one in Figure 1A have therefore become conventional wisdom in vision science, optometry, and ophthalmology ( Ciuffreda, 2006 Chauhan & Charman, 1995). Lags of 1 diopter (D) or more have often been reported even for distances that are still within the range of distances to which the eye can change its state (i.e., distances farther than the near point and nearer than the far point).

This is the lead of accommodation ( Morgan & Olmsted, 1939 Morgan, 1944, 1968 Heath, 1956 Fincham & Walton, 1957 Charman, 1999 Plainis et al., 2005). At long distances, the response is nearer than the stimulus this is illustrated by the icon in the upper left. Such an error is called the lag of accommodation.

For most stimulus distances, particularly near ones, the observed response is less than the stimulus (i.e., the eye appears to have focused to a farther distance than the stimulus) this is illustrated by the icon in the lower right of the figure. When the distance to the object producing the image (the accommodative stimulus) is varied, the resulting response follows a pattern like the one in Figure 1A. In accommodation, the eye’s crystalline lens changes its power to minimize the blur of an image on the retina.

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