. This is the case for a 4mm diameter pupil eye where the order to keep the area of prime interest of an object on the fovea. angular limit of resolution is about 0.5 minute of arc due to the The exact mechanism of vision is not fully known. The complexity diffraction patterns. may be conveyed by the fact that the cones of the fovea are The arc subtense of a cone is about 1 minute of arc SO that the connected individually on to a single nerve fibre and have direct average eye is able to resolve details subtending 1 minute of arc at path to the optic nerve. Outside the fovea the rods and some cones the eye. Even so, most people can only achieve this resolution under are multiply connected to nerve fibres which enables pattern vision. the most favourable conditions. It is important to distinguish be- The time varying signals from the millions of nerve fibres are tween seeing and resolving. The eye can 'see' a star subtending an continuously interpreted by the eye-brain combination in order to angle of less than an arc second but it cannot 'resolve' two stars construct the perception of a scene. The brain of course, does not separated by such a small angle. Defects of vision 7/11 7.8.2 Sensitivity enabled by the ciliary muscles bringing about a change in shape and power of the crystalline lens. With age the crystalline lens becomes The photopic and scotopic spectral sensitivity of the eye is shown stiffer and the available range of accommodation diminishes. The in Figure 7.2. The photopic response applies at higher levels when closest point on which the eye can focus, the near point, might be the eye is adapted to any light level from the brightest sunlit scene about 7 cm for a teenager, 25cm for an adult and around 100cm in to that lit by early twilight. Here cone vision dominates. In much older adult. The far point is the most distant object which can be darker conditions, for instance at just before nightfall, the peak of brought into focus. In a normal or emmetropic eye this distance is the eyes sensitivity has 