FOVEA

The central part of the retina, the Fovea, is composed only of cone cells. Anything that needs to be examined in detail is automatically brought to focus on the fovea. The rest of the retina fulfils the function of attracting our attention to movement and change. This is termed as Central Vision.  As light decreases, the sensing task is passed over from the cones to the rods.

=> In dim light it is easier to focus on an object if you look slightly away from it. As the fovea contains no rods, which would be required for vision is very low brightness levels, the centre part of the eye becomes blind to dim light. It is then necessary to look away from the visual target so that the peripherally located rods can perform their sensing task.