Pupil size also sets depth of field, with a smaller pupil raising tolerance for focus error

What it is

Besides controlling how much light enters, pupil aperture works like a camera aperture in setting depth of field: a smaller aperture keeps a wider range of distances acceptably sharp and tolerates more focusing error, while a larger aperture narrows depth of field so that even a small focus offset looks blurry. The upshot is that the same pair of eyes, looking at the same imperfectly focused scene, experiences different sharpness in bright light (pupil constricted) versus dim light (pupil dilated).

Why it happens

This is the pinhole effect from geometric optics: a smaller aperture narrows the range of ray paths that pass through it, so even when the image plane is not precisely matched to the object's distance, the resulting blur circle on the retina stays small and still looks acceptable; a larger aperture widens the range of ray paths, so the same focusing error is magnified into a noticeably larger blur circle and the image looks blurrier. The eye's own accommodation (the ciliary muscle reshaping the lens) rarely matches the target distance with perfect precision — there is usually a slight accommodative lag. This lag stays largely unnoticed under a small pupil because of the extra depth-of-field margin, but shows up as perceptible blur more readily under a large pupil.

Studying it

A common method fixes pupil aperture at several sizes using an artificial pupil (a mask with holes of different diameters) under the same illumination, then compares participants' visual acuity or subjective sharpness ratings across these artificial apertures; alternatively, ambient illuminance can be manipulated to bring the natural pupil to different sizes, and the discriminability of fine detail under the same accommodative error is measured. Common independent variables are pupil/artificial-aperture diameter; common dependent variables are visual acuity, sharpness thresholds, and the range of accommodative error that stays acceptable.

Where it stops holding

This relationship describes the depth-of-field margin gained from aperture changes within the same pair of eyes at the same refractive state — it cannot substitute for actual refractive correction. A constricted pupil only widens the range of tolerable focusing error; it cannot fully compensate for persistent blur caused by an incorrect lens prescription, no matter how small the aperture gets.

Applying it

• For use scenarios with large swings between bright and dim environments (reading outdoors in strong light, driving displays), expect that pupil constriction in bright light gives extra depth-of-field margin, so near-field detail can read more sharply there than in dim light — but do not assume the same font size and line weight will be equally legible in dim conditions.
• For devices requiring prolonged close-range viewing of fine content in dim light, set the minimum legible size for text and icons based on sharpness thresholds measured under dim, large-pupil conditions rather than reusing numbers measured under bright light.
• Verification: test the legibility or recognition accuracy of the same interface elements under both high and low illuminance, to check whether the design is over-relying on the depth-of-field bonus that bright-light pupil constriction provides.

Related

• Same group: A1.20.1 Pupil diameter automatically contracts and dilates with ambient light to regulate incoming light · A1.20.2 Pupil adjustment is slower than the brightness jumps electronic displays can produce · A1.20.4 Pupil response is also driven by cognitive load and emotional arousal, not light alone · A1.20.5 High-contrast scenes create competing demands on pupil size, producing local over- or under-exposure
• Nearby: A1.21 Lens accommodation and vergence · A1.03 Visual acuity and minimum discriminable detail
• Site search: pinhole effect · depth of field · pupil size · accommodative lag