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The blind spot is where the optic nerve leaves the eye and there are no receptors, either rod or cone. Its not a problem with binocular vision as each eye compensates for the other, but can be a problem in monocular vision, but only off center as the center of our vision, the fovea (aka the macula), is rich in cone cells. So the blind spot can become a problem when we use averted vision. The reason for using averted vision is that the fovea has no rod cells, only cones and is therefore not useful for low light vision (as described in previous posts). Rod cells are located away from the fovea so averted vision helps when light is low. When I was studying this stuff in physiology some 25 years ago, there was some evidence that a fully dark adapted human rod cell will respond to 1 photon of light in the visual spectrum, provided it isn't red light. If 2 rods within 10' arc go off, it will be perceived. This is equivalent to seeing a candle at 30 km. I don't know if more recent work has confirmed this. Dark adaptation is the replenishing of rhodopsin, the pigment that absorbs light and triggers the electrical activity in the cell. When a bright light is seen, the rhodospin is exhausted and the cells will take up to 40 minutes of darkness to restore it fully.
Patrick
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