he couldn't read and he lived in a wealthy enough upbringing to try elixirs, tutors, and a ton of other bullshit, so it only makes sense that jan van dick tries prescription glasses somewhere along the way. they don't help him read (for obvious reasons), but they do fix some of the eye issues he hadn't known about before so he sticks with them. when he leaves for school (cough cough almost gets killed by his dad's hitman), his glasses break/get lost or he has to sell them for a little extra kruge and he just learns to live with bleary eyes and blurry features
fast forward to the end of crooked kingdom, he's back in the van eck mansion and after all the crazy dust settles he realizes, 'hmm, it would be pretty nice to see about now!' and he buys/finds/acquires glasses. hooray, now he can see again! except, uh oh, he forgot to mention to the rest of the crows that he's a four-eyes and suddenly all of them are staring at him, slack-jawed because 'you went through all of this and you couldn't even see????'
oh yeah, and jesper goes crazy because those are some slutty glasses