Figure 7.

The subjective near point of accommodation (black circle) for an observer with natural lens, derived from the blur-circle (PSF) diameter (left ordinate) in a simplified model. Moving along the solid blue trajectory in the graph from right to left corresponds to an approaching target. With proper accommodation, no dioptric blur occurs (blue line); acuity in this condition would be referred to as the distance acuity. Proximal to the limit of accommodation (here assumed to be at 2D as in a typical 50-year-old emmetropic observer), dioptric blur increases and degrades acuity (solid red line). The dashed red line represents the blur-circle diameter in the case of a fixed focal length, as in a person with a monofocal intraocular lens, assumed to be in-focus at 50 cm.