The retina in humans has the light receptors placed behind the nerve pathways. Light must pass through a tangle of nerves before it is detected.
How much does this diminish our vision?
What problems does the arrangement produce?
What are the major groups with which we share this arrangement?
Are there any animals which benefit from the reverse organisation i.e. nerve receptors placed before the nerves leading to the brain?

Hi Roger,

the retinas for all vertebrate eyes tend to follow this pattern (arthropods have a different arrangement with pigment detectors and light detectors at the end of a long cone and before the rest of the nerves).

I don't think the arrangement is actually particularly problematic. Incoming light levels can be quite high, and although the pupil can reduce the total amount of light getting in, the intensity can still be quite harmful to the highly sensitive light receptors. The nerves in front of the retina act a bit like internal sunglasses, reducing the intensity of the light. In nocturnal and crepuscular animals this light dampening effect is offset by the tapetum lucidum, which is a reflective layer behind the retina that reflects the incurrent light back onto the light receptors (making cat's eyes etc. reflective). The fovea is a small area in the retina that has fewer cells in front of the light receptors (particularly cones for colour determination in humans), this is where high visual acuity occurs. The rest of the retina tends to deal with peripheral vision - light/dark and motion detection. The brain ties these different functions together to provide a single unified view of the world. There is also a blind spot on the retina where the optic nerve enters, this spot is not very noticeable in our vision very often, because of the way the brain fills in gaps in the images received from the eyes.