IYDK, the actor was born with a lazy eye, which naturally corrected itself, but he's been rocking strong corrective lenses since he was 6 -- and for him, not being able to see clearly has always been the norm.

In fact, Jake says his vision woes gave him an acting edge ... for example, he tells The Hollywood Reporter he ditched his contact lenses for his 2015 action flick, "Southpaw."

To make his character's reaction to learning his wife died more realistic in one scene, he didn't wear his lenses so he could only react to hearing the news delivered to him.

As for his personal life ... his blurry mornings have become a sanctuary from the chaos, offering a peaceful escape before he puts his glasses on.

The American Foundation for the Blind (AFB) says 20/400 vision is considered very low functional vision ... and Gyllenhaal’s vision is 20/1250 -- so yeah, that officially makes him legally blind.

This isn’t Jake's first time opening up about his vision issues ... back in 2017, he shared he was bullied in school for wearing corrective glasses.