Brief Answer:
in more serious cases, related to the retina at the back of the eye

Detailed Answer:

Hi,

Floaters in the eyes can be due to serious causes in some persons whereas it can be related to minor and resolve on its own.

If he had the floater for a long period of time and it is not getting worse or affecting his vision then it is possible that there are no problems.

If your dad has a history of problems with his retina which is a layer at the back of the eyeball, then it is possible that the floater may indicate severe change in the retina that need to be assessed.
He would know this if he has regular eye exams.

If the floater started suddenly, is getting worse, is affecting his vision or seeing the floater occurs with flashes of light- then he should be seen as soon as possible to check the retina and other areas of the eye.

I hope this helps, feel free to ask any other questions

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