Visual resolution when light enters the eye through different parts of the pupil.

1. Threshold contrasts for resolution of sinusoidal gratings imaged on to the retina through a decentred 2 mm pupil were measured.2. No loss in resolution was found when the pupil was decentred parallel to the lines of the gratings. A loss in resolution by a factor of 3 occurred when the pupil was decentred by 3 mm perpendicular to the lines of the gratings.3. The effects of focus on the threshold contrast for a grating viewed through a centred and decentred pupil were used to show that at least a portion of the loss in resolution is due to optical aberrations.4. Using a neon-helium gas laser as a coherent light source, interference fringes were produced on the retina directly. Threshold contrasts for resolution of the fringes were determined for different positions of entry of the beams of light through the pupil. When the Stiles-Crawford brightness effect was compensated for, no loss in resolution was found to occur for decentred entry of the beams.5. It is concluded that the off-axis loss of visual acuity is wholly due to optical aberrations in the eye.6. The ratios between the threshold contrasts for sinusoidal gratings and for interference fringes are used to calculate the optical transfer functions of the off-axis aberrations of the eye.