Figure 1.

The viewing geometries underlying blur and disparity. A) Formation of blur. A lens is focused on an object at distance d₀. Another object is presented at distance d₁. The aperture diameter is A. The images are formed on the image plane at distance s₀ from the lens. The image of the object at d₁ is at best focus at distance s₁ and thus forms a blur circle of diameter b₁. That blur circle can be expressed as the angle β₁. B) Formation of disparity. The visual axes of the two eyes are converged at distance d₀. The eyes are separated by I. A second object at distance d₁ creates horizontal disparity, which is shown as the retinal angles Φ_L and Φ_R.