intensity difference during movement

Brain can judge surface relative distance by intensity change during movement toward and away from surface {intensity difference during movement}. See Figure 1.

moving closer

Moving from point to half that distance increases intensity four times, because eye gathers four times more light at closer radius.

moving away

Moving from point to double that distance decreases intensity four times, because eye gathers four times less light at farther radius.

moving sideways

Movement side to side and up and down changes intensity slightly by changing distance slightly. Perhaps, saccades and/or eyeball oscillations help determine distances.

memory

Experience with constant-intensity objects establishes distances.

accommodation

Looking at object while moving it or eye closer, or farther, causes lens-muscle tightening, or loosening, and makes more, or less, visual angle. If brain knows depth, movement toward and away can measure source intensity.

light ray

Scene points along same light ray project to same retina point. See Figure 2.

haze

Atmospheric haze affects light intensity. Haze decreases intensity proportionally with distance. Object twice as far away has half the intensity, because it encounters twice as many haze particles.

sound

Sound-intensity changes can find distances. Bats use sonar because it is too dark to see at night. Dolphins use sonar because water distorts light.