Suddenly Intermittently, I see a black spot of light in vision they don't last for a long time, perhaps 3 -4 seconds and then disappear, they're basically black spot of light that has a glowing ring around it , as it was possible od retinal tear , I saw an optometrist at Specsavers who did an eye exam and checked my retina and couldn't find a cause , I don't have a retinal tear it at least that's what my optometrist said, it was a good optometrist , are the black spots of light in vision serious, they don't last for a long time and I have mo other symptomsshould I be worried about this ?