Many of the Pixar animated movies have a few catch phrases like "I'm okay!!" or "My Eye!!" Having only been to an eye doctor once before, years ago, to find out if there was any reason to be concerned about "floaters," today I went to find out whether there was anything more to the blurry focus in my right eye than just the ordinary effects of getting older. I was relieved to learn that "I'm okay!!"
I learned a few more things about eyes too that I might have figured out, based on what I know about photography, if I'd given it any thought. For the few weeks prior to the eye exam today, I had been checking each eye under different conditions so I'd have more useful ways to describe how my right eye was bothering me. One thing I had noticed is that in lower lighting, one of my eyes seemed to give me a darker image but the same wasn't true in the brightly lit outdoors. The other thing I noticed is that looking at certain things like a television or the moon seemed to cause double vision worse than other things like a shelf full of small objects or dishes on a table.
So, as it turns out, my right eye is just not focusing quite right any more at a distance. However, that didn't readily explain the comparative brightness in low light only. If I had thought a bit about it, I would probably have remembered that to increase the depth of field, i.e. expand the range of things that are projected sharply on a camera's back-plane, the aperture can be made smaller. This lets less light through, so the projected image, although sharper, is also darker. With a camera, a brighter flash can offset the darker image but with an eye, the available light is fixed. Apparently, one of the things the eye will do in order to compensate for the failure of the lens shape to project a sharp image on the retina is to constrict the pupil to increase the depth of field. So, like a camera with the aperture narrowed, less light gets to the retina. It makes sense that blurry results in darker, but it isn't really obvious.
Double vision puzzled me because it seemed to only appear when I viewed things that were far away, somewhat high contrast, and, to the naked eye, essentially 2D / flat, like TV or the moon. I probably learned in grade school that the image projected on the retina is actually inverted up/down left/right from the scene that appears before the eye. The brain does the work of flipping everything back around so it all makes sense and you don't end up reaching low and to the left to catch a ball passing high and to your right. The brain also compensates a little when your eyes don't quite line up with one another. That's easy for 3D objects that are up close because the actual image differs enough that the brain is composing a 3D perception anyway. When the brain is attempting to align the distant image from two perfectly focused eyes it's easy to match up edges too, but when one of the eyes is sharp and the other is blurry, the brain-alignment doesn't work as well. The result is that the brain partially surrenders the task of compensating for what is perceived as physical mis-alignment of the eyes, and delivers a halo or a double image. This also makes perfect sense if you consider the challenge of overlaying a blurry image and a sharp image in a graphics program like Photoshop. It would be almost impossible to determine the exact positioning with crisp edges upon which to align in only one of the images.
So this simplified explanation is for my own benefit later, when I've forgotten what I came to understand today, and it's here in case someone else stumbles over it and finds my particular way of explaining it easier to understand than some other way.