Their eyesight is very poor.
This is something of a myth. Hogs have eyesight that isn't too far off from humans in the zone where their vision overlaps (~50 degrees of frontal field of view). Beyond that, they have reasonably good peripheral vision like other similar animals, which accounts for more of their vision, but is undoubtedly better than a human's peripheral vision. They are red-green colorblind like many other animals, but see farther into the UV spectrum than do humans. They also see at night better than humans.
If you are familiar with hog calling, Glenn Guess is a big name in that area. Glenn really turned me on to the notion that hogs see well. He raises hogs and experiments with them, harvest sounds from them, etc.
I have watched hogs run 20-30 mph through the woods at NIGHT after being shot at and NOT run into trees. Try that with a human. The hogs are not smelling the trees to avoid them. There is no indication that they use echolocation either.
I am convinced people believe this popular myth because of hog behavior. They feel that if they can approach hogs, that hogs must be blind, otherwise they would run away, right?. However, physical blindness is not the issue.
Part of the issue is their low FOV, low eyes and head position relative to higher grass.
Part of the issue is behavioral, hogs having little to fear in terms of predators anymore...other than humans and they have to learn that.
Another part of the issue is what is called inattentional blindness. Just because something is in the field of view of the animal does not mean it will be noticed so long as the animal's attention is on something specific. It has been studied in humans and there has been some related testing in animals.
https://www.smithsonianmag.com/science-n...dness-17339778/ The point here is that if you have a hog focused on seeing kernels of corn on the ground, rooting tubers, etc., it is more apt to miss you walking up on it than if it is just walking through a field.
It is important to keep in mind that their peripheral vision is not nearly as good as their binocular vision, but that their binocular vision is very good. If you can approach a hog from the back side, its ability to see you will be greatly diminished. Approaching the hog laterally is less ideal, but that this the location from which most hunters like to shoot. Probably the least idea direction to approach from is the front.