Is this just stating the obvious or what? LOL!
The contrast between a dark house and a sunny day becomes more obvious when people go outside for a while and then go back inside.
The contrast between a good food and bad food becomes more obvious when one eats a lot of good food for a while, and then eats some more bad food.
One could go on forever!
This is not true. I've worn glasses for several years now and if I take them off, my vision is much worse than it was before I started wearing glasses, not just in contrast; I now see nothing but blurs. I don't believe the glasses are "weakening" my eyes either; it's just that my eyes have gotten used to seeing with them rather than straining to see without them. It's certainly more than just the contrast though; there is a definite change in my non-corrected vision.
I think it was just a poorly constructed sentence. Most people who have just switched to corrected vision didn't think their vision was that bad before hand (unless they really put it off for too many years), but then after a few months of wearing glasses they take their glasses off and suddenly WHAMMO they're freaking blind. The gut response is to think you're eyes have gotten worse, but the reality is that you've gotten used to being able to see again and by contrast where you were before (which you got to gradually and largely without noticing) seems terible. I know that's what happened to me, after 6 months with the glasses I suddenly found I couldn't read without them even though I could before but with strain, nothing had happened to my eyes I just had gotten used to corrected vision and the vision center of my brain was lodging its protest against going back.