What is the difference between depth perception and stereopsis?

That’s a good question. Stereopsis (when the brain perceives depth by interpreting the visual input of both eyes) is determined solely by the two eyes working together to develop a three-dimensional image. Depth perception is partly determined by the degree of stereopsis.

However, there are monocular clues to depth perception also. In the extreme example of someone who has only one functioning eye, that person will have no stereopsis. But they would have some depth perception from monocular clues. Examples of monocular clues include: image size disparity, i.e., images further away appear to be smaller; motion parallax, i.e., images further away appear to move slower; and blocking, i.e., if one image blocks the ability to view another image then you know that the image doing the blocking is in front of the image being blocked. There are other monocular clues to depth as well.

In summary, to have any degree of stereopsis you need to have two functional eyes both looking at an object. This ability would contribute to the “binocular clues” to depth perception. However, one who has no stereopsis at all can still have some degree of depth perception due to monocular clues to depth.