The capacity to concentrate is one of the primary distinctions between human and sheep eyes. Sheep lack a fovea in their retinas and hence cannot focus as clearly as humans. The position of the eyes in humans and sheep is also different. Sheep have eyes positioned on both sides of their heads. Humans have one main eye located in the center of our faces. There are some other differences as well, but these are the most significant. Sheep's vision is generally worse than that of humans because they don't have anything like our deep-focus ability to bring objects close up while still seeing them clearly at a distance. This is one reason why shepherds need to be able to see distant objects as well as those coming right at them when herding sheep or cattle. Humans can focus on multiple things at once, while sheep can only focus on one thing at a time. This difference comes in handy when trying to train sheep to accept people handling them carefully without panicking. Since people can handle more than one animal at a time, it isn't difficult to get several sheep to accept someone who is only touching each one for a few seconds at a time. This would not be possible with just one human being and one sheep. Sheep's eyes are also less protected than ours, so they suffer more from dirt, dust, and other contaminants.