If the receptors were at the front of the retina, the pigment cells would have to be between them and the next layer of nerve cells, in a region already packed with axons, dendrites, and synapses. As it is, the layers in front of the receptors are fairly transparent and probably do not blur the image much.
The definition of impervious is not being able to be penetrated or not able to be hurt. 1  An example of impervious is a fabric that does not allow any liquid to pass through. 2  An example of impervious is a figurine made out of indestructible material.
Medical Definition of OPACITY. 1. : the quality or state of a body that makes it impervious to the rays of light; broadly: the relative capacity of matter to obstruct by absorption or reflection the transmission of radiant energy (as X-rays, infrared radiation, or sound). 2. the quality or state of a body that makes it impervious to the rays of light; broadly: the relative capacity of matter to obstruct the transmission of radiant energy. 3. : an opaque spot in a normally transparent structure (as the lens of the eye). See opacity defined for English-language learners.