The other day I was sitting in my lounge alone and my dog got up and started growling and barking at the air. She wouldn't stop whining and wouldn't take her eyes of this spot for ages.
Can dogs see things that humans can't?
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Yes and no. That is, in some areas they see better, and in some areas they see worse.
Both human eyes and dog eyes have rod and cone cells making up the retina. Rods are better at vision in low light conditions and detecting motion. Cones are better at visual acuity and can actually detect color.
In comparison to humans, dogs have more rod cells, which means their visual acuity in well-lit conditions is actually poorer, and of course their color vision is much worse. Their vision is usually estimated to be 20/75 versus 20/20 which is considered the norm for humans, although certain types of dogs, such as sight hounds, may have vision acuity closer to that of a human's.
On the flip side, they will see better under low light conditions. In addition to their rod cells, unlike the human eye, a dog's eye has a layer of tissue behind the retina called the tapetum lucidum. It's a reflective layer that reflects light back onto the retina, to help increase the light that hits the retina. This improves their night vision even more, and it's what gives dogs that distinctive greenish yellowish eyeshine in flash photos.
Furthermore, a dog's eyes are set a little more to the sides of the face than a human's, giving them a narrower field of good binocular vision but a little wider field of vision overall. They can compensate for the lesser binocular vision by turning their head to get the thing they're trying to look at more centered in their vision.
But also in comparison to humans, dogs are able to detect higher frequencies of flickering light, whereas to a human a light flickering so quickly will appear steady. In other words, dogs have a higher flicker fusion threshold. This means that it takes more frames per second in video media for the image to appear to be smoothly moving to dogs than to a human. Television, which is optimized for the human eye, may very well look like a stream of flickering still images to them. This is especially true of older media, where framerates tended to be lower.
To summarize, a dog's vision:
I see that this was asked a long time ago but it's still worth answering.
Dogs are predators and are very sensitive to movement. They can sometimes miss seeing a stationary animal (such as a rabbit) when it is obvious to us. However the second it moves, they will spot it.
Dogs with good eyesight can see tiny flies such as fruit flies in the air, even in conditions where it is hard for us - again this is because the flies are constantly moving.
I regularly observe this with my own dog. She will chase and snap apparently at mid-air. It is only occasionally that I will manage to see what she sees but I certainly do sometimes if I look where she is looking.
So yes. Dogs who have good eyesight, can see things that humans can't. There's nothing weird about it. They are just more sensitive to movement. Also it is quite possible they hear things that we don't and your dog was looking in the direction of a sound.