Vision correction affects proprioception: objects in focus are not where they appear to be. Apparently, only some people—perhaps especially those with an impaired cerebellum, such as myself—are particularly sensitive to this 3D shift—i.e., the discrepancy between where an object actually is vs. where the cerebellum computes the object to be based on what the distorted eyes are mistakenly telling it. If you are touching the object, then the cerebellum is doubly confused by conflicting data from your eyes and your fingertips.
I can only assume that having multifocal or progressive lenses would wreak further havoc with proprioception. (I have not tried them.)
Here’s a short list proprioceptive issues caused by the distortions of vision correction:
- Looking at the ground while walking is disorienting. The ground appears to be at a distance different from what my legs report. This makes taking steps, and dealing with small gradient changes, error-prone and dangerous.
- The process of sitting down in a chair reveals that I’m unsure of exactly where the chair is, even if I look right at it as best I can.
- Dealing with doors, including refrigerator doors and cabinet doors, is disorienting, as they swing through an angle that doesn’t match what I see.
- Without going into too much detail, typically anything involving using a bathroom is imprecise and troublesome.
In short, it makes the world feel like a stage designed to embarrass me.
I know I am, but what are you?
I am nearsighted, which means that without correction, I can only focus on near objects. In addition to being nearsighted (AKA having myopia), in 2017 at age 49, I think I have a bit of presbyopia, which means I cannot focus without correction on objects that are very close; I can only focus at a specific near distance. Currently, I can focus without correction on objects that are about 7.5″ (19 cm) from my eyes. As that distance grows, it won’t truly become a practical problem without correction until that distance approaches and then exceeds arm’s length.
If I wanted to focus on objects closer than 7.5″, I would need correction with a positive (magnifying) optical lens power (convex in shape), though I have not experimented with that beyond using a simple magnifying glass in isolated cases, such as to help remove splinters from my fingers. To focus on things farther away, I use glasses with a negative optical lens power (concave in shape); i.e., they shrink the image.
In terms of focal points
When looking at a distant object, my uncorrected eyes focus the image in front of my retinas; to move the focal points back to land on my retinas, I need negative optical lens power.
My uncorrected eyes focus the images of extremely near objects behind my retinas; to bring the focal points forward, I would need positive correction (magnification).
When I put on glasses, distant objects I see are not blurry but at the cost of shrinking them. Shrinking causes the world to shift (out, up, and to the right, for my prescription—I don’t know why the multiple shifts), and it’s impossible for my impaired cerebellum to compensate and compute where things are based on where they appear to be; it cannot correctly compute where I or parts of my body are within the 3D world of my existence. I assume most people’s cerebellums compensate adequately to compel them not to complain, thus making me sound like in inconsolable whiner, bordering on hypochondria. I have been dealing with this since 1991 at age 23, now 26 years ago.
Furthermore, if I use my peripheral vision (i.e., look indirectly outside the lenses sitting in front of my eyes), near objects are in focus but distant objects are blurry and exist in a shifted 3D space. Again, my impaired cerebellum comes into play. The near world exists in a 3D space that is shifted slightly from the world in which the far world exists. For a few corner cases on the edges of my glasses, I see two versions of the same thing: one blurry and one in focus, where one version is shifted slightly. If I’m moving, such as walking or riding a bicycle, this is utterly irreconcilable, as not only do I see two shifted 3D worlds, but they are both moving.
Note that ignoring peripheral vision does seem to be learnable, but compensating for a distorted proprioception does not (in myself, anyway).
I used contact lenses for a few years, only when on my bicycle, but I no longer ride due to SCA3 issues. To be clear, they did not affect the proprioceptive shift of vision correction.
For me with SCA3, the main benefit of contact lenses was improved peripheral vision. Contact lenses eliminate the peripheral vision problem that glasses have, which for me (with SCA3) was essential. I wore contact lenses when on my bicycle for multiple years because of this, coupled with clear-lens sunglasses as a wind shield.
The main drawback of contact lenses (not related to SCA) is that while wearing them I would lose the ability to focus on near objects; e.g., I could no longer focus on my cyclocomputer display. Wearing glasses with a mild prescription never affected my ability to focus on close objects. If glasses were to interfere with my vision, I could easily temporarily pop them off, but that’s not a realistic option with contact lenses.