As the title states this deals with the human eye. I am not claiming to be an expert on camouflage, but simply explaining how the eye operates.
Here is a good shot of the eye's internal workings:
The eye is very complex, but we are only concerned with the most outer parts.
The lens
The lens of any eye is a convex shaped wide angle lens, and also note that the cornea is also convex. This causes the most center of our vision to be properly focused, and the rest of our viewable area to be poorly adjusted for nomal viewing. I can't just dub these ill focused areas useless; they are very apt at detecting motion.
Nervous link
As with any body part, the eye contains nerves. But unlike any other major body part, the eyes are located the closest to the brain (intrestingly, the optic related portion of the brain is located in the rear). The eyes are also probed the fastest, thus having the fast refresh rate delievering that quality HD we enjoy naturally. But there is a trade-off, optical memory retension is very low. The brain cannot store the vast amounts of memory the eye could use, so it is just dumped. However, anything in any part of your field of view will continue to remain in memory.
The Brain
The brain is the driving factor behind the eyes. It takes all the information available and analyzes it. It then seperates the 3D image into different layers, and defines different objects using depth perception, color, and contrast. The brain then computes and determines what each object is. We can percieve the difference between food and poo, a door and a window, and wood vs. steel.
These are not hard concepts to understand and could make all the difference in the field. Happy hunting.