Advantages of a Mobile Pupil

The normal human pupil can change diameter from 8 to 1.5 mm, which corresponds to approximately a 30-fold change in area and almost a 1.5-log unit change in retinal irradiance. Although the visual system can operate over a 10-log unit range of lighting levels through the process of adaptation, it can take several minutes for optimum sensitivity to return after an abrupt increase or decrease in retinal illumination. The rapid control of retinal irradiance by the iris allows the visual system to more quickly regain optimal sensitivity by dampening fast changes in ambient lighting levels and by requiring less retinal adaptation for a given change in environmental lighting levels.

However, changes in pupil size affect not only retinal illumination, but also diffraction, optical aberrations, and depth of focus of the eye. These factors differentially affect visual performance and, given changing environmental lighting conditions and visual tasks, the nervous system continuously modulates pupil diameter for optimal visual performance.

The diffraction of light rays by an aperture is a major limiting factor in the resolution of an image in any optical system. The amount of disruption in image quality caused by diffraction at a circular aperture decreases as the size of the opening increases. Therefore, as pupil diameter increases, there is decreased degradation in retinal image quality caused by diffraction. In contrast to diffraction, the image-degrading effects of optical aberrations increase as aperture diameter increases. Therefore, as pupil diameter increases, the degradative effects of optical aberrations also increase, and offset the benefits gained by reduced diffraction at larger pupil diameters. Over the normal range of pupillary diameter, diffraction impacts image quality less than optical aberration, and the optimal pupil diameter is therefore approximately between 2 and 4 mm.

Along with diffraction and optical aberrations, defocus is an important determinate of retinal image quality. Although the pupil does not refract or focus light, it influences the depth of field of the eye. Depth of field is the range of distance in depth in which objects appear to be in focus. For example, when one reads a book, the power of the crystalline lens of the eyes changes in order to bring the text on the page into focus through a process called accommodation. With the eyes accommodated on the book, all objects within a range in front of and behind the book will also appear in focus. This range is called the depth of field and it is primarily dependent both on viewing distance and pupil diameter. When the viewing distance is held constant, the depth of field increases with decreases in pupil diameter, and therefore the pupil diameter can affect the focus of the retinal image.

Clearly, a mobile pupil allows the nervous system to optimize retinal irradiance, diffraction, ocular aberrations, and depth of focus despite differing conditions and visual tasks. For example, across a range of daylight (photopic) luminances, pupil size corresponds to that required for the highest visual acuity, and the maximal information capacity of the retinal image. On the other hand, under low light (scotopic) conditions in which poorer retinal image quality can be tolerated due to the lower resolution of rod photoreceptors, the pupil dilates sufficiently to maximize the retinal illumination. Further evidence for the optimization of pupil diameter for differing visual tasks is evident in the pupillary near response (PNR). When the viewing distance changes from far to near, the pupils constrict to increase the field of view and reduce the retinal image defocus. This compensates for the decrease in the effective field of view that naturally occurs when viewing distance decreases (see the section titled ‘Pupillary near response’ for more details).

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  • Category: Eye diseases