### Cues that help us determine how far away objects are.

What factors do you suppose influence whether one object appears closer than another? Think about if for a minute. Before we proceed to discuss the perception of distance it would be a good idea to look at the photo

When you look at the photo see if you can come up with several pieces of information that cause you to say the man is near the front and the buildings are way in the back. Is the fence near the man parallel to the frontal plane or does it recede away?

I will displace the rest of the text down a few lines to encourage you to look at the photo before you read the following. Now is a good time to click on the photo. To return to this text, just click anywhere on the photo.

Did you notice that the man (1) overlapped a number of the elements in the picture that appeared further way than he did? The man is also larger than objects which are in the distance. Compare, for example, the size of the man's head and the size of the windows in the building (4) across the bay (3). Clearly if the man was in one of those buildings looking out of the window his head would be much smaller than the window. But his head being closer is much larger. The man's body also overlaps the water which means he is in front of it.

Look at the building labeled (5) . It would appear to be further way than the white building (4). The white building is larger and overlaps the middle building. However, the tower (6) is taller than any of the other objects in the photo. Still it seems far away on the other side of the bay. We know that the tower is behind the man because his head overlaps it. If you look closely you will also notice that the image of the tower is seems a little less clear than things which are near the front. This is because of a little haze in the air. The further away objects are the more atmospheric haze one must look through.

Take a look at how the elements of the photo progress from the bottom to the top. The things that are further away from the front tend to be higher up, do they not? Now look at the fence (7). Notice how the left side is a little smaller than the right? That is because the fence is not parallel to the frontal plane. The left side is further away from the front of the scene than is the right side.

There is at least one other factor that provides distance information. Look closely at the water (3). Can you see some ripples in the water's surface near the front? But at the rear of the picture it is difficult, if not impossible, to see any ripples. The reason is that while there are ripples all over the surface of the water, they are approximately all the same size. But the further the ripples are from the viewer the smaller they are on the retina. To say it technically, the visual angle subtended by the ripples gets smaller the further away they are. A similar situation exists with the pattern on the surface (2) on which the man is standing. The elements of the pattern near the front are larger than those nearer the water. This phenomenon is also referred to as texture gradient and it is a powerful clue to distance.

Another way to consider the condition that the further away objects are the smaller their visual angle, or the smaller their image on your retina is with a perspective illusion. That the lines radiate to some common point in the distance is called perspective. Because the rectangular blocks are actually all the same size, the ones further away seem larger because the conflict with the perspective information.

The fact that we have two eyes (binocular vision) also provides information on distance. One of these binocular distance cues is called convergence. Convergence refers to the turning in of our eyes as objects come closer to our eyes. The other thing that happens as objects come closer is that our accommodation changes. There is a change of focus that occurs when the lens gets fatter for nearby objects.

Another cue to distance perception especially for more complex scenes in which there are multiple objects is binocular disparity.

Yet another cue to distance is motion parallax.   As you move from one location to another objects at various distances will move in a direction dependent on where you are fixating.

Finally, it turns out that color and brightness can have an effect on how far away something appears.