While relatively short LEO flight eye problems can be fixed by an earthly eye doctor, there are none on Mars. Changes are worse over time. Long duration trips in years like to Mars would have them arrive effectively blind.
But exactly how? I need to some research.
One of the few time I saw my dad cry was the moon landing. We went to see Apollo 11 parade in Chicago.
Find space travel interesting and since I do eye stuff for a living, this gets me curious.