Healthy or heathful?

First of all, I have a huge question that needs answering. It’s not a hard vocabulary question but sometimes it bothers me a lot. It would be excellent if someone could tell me how to differentiate “healthy” and “healthful”. It’s very important since this word comes up quite often in my exercises about word forms.
For example: These fruits and drinks are healthful for our bodies. Why don’t we use “healthy”?
Secondly, could you give me the correct tense of the verb below?
If you read in bad light, your eyes (ruin). Is it “will be ruined”?
Thank you in advance!

Courtecy of

I think healthful is an old fashioned word and used mainly to describe food. Now, healthy can be used to describe not only physical quality but also healthful food.