Double vision is when a person sees two images of a single object either some or all of the time.
The two images may appear on top of one another or side by side, or sometimes they may experience a mixture of both.
The charity says: “This is often caused by problems moving the eyes.”
It adds that some people with Parkinson’s experience “tracking”.
“This is when the eyes do not move smoothly across a line or from one object to another,” it explains, “for example moving across a page when reading, or up and down.