I can see clearly now...

My glasses frames (should "glasses" be possessive, or is "glasses" being used as an adjective here?) are almost 4 years old, and my lenses are a bit out of date (one lens is a full diopter off), so my mom and I went looking for new ones today. Which brings me to one of my questions (or another, if you count the possessive/ adjective question):
If the brain can auto-correct for an upside-down world or everything off by 15 degrees (with special glasses), why can't it auto-correct my vision? Is it a physical, optics-related issue? Does the brain just not realize your vision is off? Is it something else?