Physiological Optics and Visual Perception
Physiological optics and visual perception
This domain tests how the eye forms an image and how the visual system converts it into detectable contrast, acuity, depth, and colour. Calculations often involve diffraction, visual angle, and psychophysical thresholds.
Core rules
- The Airy disk radius is θ = 1.22 λ/D radians. Smaller pupils improve optical quality until diffraction dominates.
- Snellen acuity is based on the minimum angle of resolution: 6/6 means a 5 arcmin letter whose critical detail subtends 1 arcmin.
- Contrast sensitivity is not the same as acuity. The CSF peaks at intermediate spatial frequencies, roughly 3-6 cycles per degree under photopic conditions.
- Weber contrast is ΔL/L. Dark adaptation follows a rod-cone break; scotopic vision is rod-mediated and colourless.
- Accommodation amplitude declines with age; depth of focus increases with small pupils and lower spatial frequencies.
Worked example
Estimate the diffraction-limited angular resolution of a 2 mm pupil at 550 nm. θ = 1.22 × 550 × 10^-9 / 0.002 = 3.36 × 10^-4 rad. Convert to arcminutes: 3.36 × 10^-4 × (180/π) × 60 ≈ 1.15 arcmin. This is close to the 1 arcmin critical detail of 6/6 acuity, showing that a 2 mm pupil is near the diffraction limit for daylight vision. A 6 mm pupil gives about 0.38 arcmin diffraction limit, but ocular aberrations usually prevent that theoretical gain.
Traps
- Confusing visual acuity with contrast sensitivity.
- Using pupil diameter in mm without converting to metres.
- Assuming larger pupils always improve resolution.
- Forgetting that Snellen fraction denominators are not linear acuity scales.