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To understand why this happens think of your brain as a developing house that is actually two sites set next to each other. (The brain can generally be thought of as two separate halves with the right side dealing with the left side of the body and vice-versa).
Any light coming from the left hand side of straight ahead red arrows will fall on the right hand side of the retina (in both eyes). This triggers a response from the retina causing nervous impulses red shading to be sent to the right hand side of the brain. This is achieved by mixing the optic nerves from both eyes just behind the eyes and re-organising them to give a right / left split to go to the brain. Light from the right side of straight ahead is shown, by the blue colouring, going to the left side of the brain.
When a stroke damages the vision this is because there has been damage to an area of the brain dealing with the vision. But the damage always affects just one side (it does not cross the mid-line). Thinking back to the two sites of the developing house we can consider that the same effect would be caused if there were a strike by the workforce in one of the sites.
When the nervous impulses (negatives) are delivered to the right hand site, there is nobody there to process the information and so no picture is seen. To the person this would appear as if there was just darkness to the left of straight-ahead. The information given to the other side of the brain would be processed normally giving a good / normal vision to the right of straight-ahead. With strokes some people recover quickly and fully whilst others do not seem to recover as much or at all. This seems related to how bad the original damage to the brain was and how quickly the oxygen supply was restored.
If there is going to be a good improvement in the vision after the stroke it will happen fairly soon. As a general rule, any field-of-vision loss that is still apparent after a few months is unlikely to repair.