Reading the chart perfectly tells us one narrow thing: how well you resolve high-contrast black letters on white, at six metres, with one eye at a time.

That is a genuinely useful number. It is also nowhere near the whole picture. It says nothing about how your two eyes work together, whether your focusing system holds up over an eight-hour day, what your intraocular pressure is doing, or what the back of your eye looks like.

A vision screening at work or school checks the chart. A full exam checks the eye. Those are different appointments, and only one of them will find the early pressure change, the retinal thinning, or the vascular sign that matters five years from now.