Pupil size

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Finally, we must discuss the adaptation of the pupil. The change in its size, which occurs in parallel in both eyes, serves to regulate the light entering the eye at luminances between 10² and 10³ cd/m². It only controls retinal illuminance in the 1:16 ratio. When light enters the eye, the iris muscles contract abruptly, allowing less light to reach the retina to prevent glare. Conversely, dimming causes the pupil to dilate. The control of the pupil reflex is unconscious. If it fails, this is a clear indication of serious brain damage or death.

If we look at a landscape in front of us in daylight, which contains both dark shadow areas and a bright sky with clouds, we do not grasp it „with one look“, but scan it through the involuntary saccadic eye movements, so to speak. We look successively into the shadows, into the middles and into the lights, and thus the visual system has the chance to adapt its sensitivity to the respective average brightness. When we look into the sky, the pupil closes, and lateral inhibition acts strongly. If we look into the shadows, the pupil opens, and the inhibition drops to perhaps 0. With each individual gaze, we have a maximum contrast of 1000:1 at our disposal, which expands to, say, 1,000,000:1 by combining the sensitivity adjustments. The brain builds the overall impression from the individual images without us noticing much of it. Consciously, we perceive „one image“ that is sharp from front to back and has details in the highlights and shadows. We can imagine this as if we set the exposure metering of a digital camera to center-weighted and „scan“ the different bright areas of a high-contrast motif. The small LCD screen would then show a brighter or darker image depending on the viewing direction.

Next The minimum size of brightness differences

Main Contrast

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