Why does darkness engulf vision when you stare while covering one eye?

[Harveyclues]

I believe a post made before mine had the same question, yet the young man didn't write it so well, although after usually checking these forums, I get abit panicky and need-to-know, anything to the question. We all know staring blankly in the dim/darkness your vision begin to go completely black, then dissipates upon the smallest movement. My question is, for example leaving both eyes open, cover one with your hand (do not close the eye itself), so your pinky is running down the side of your nose, then stare at something bright, like a computer screen/tv/blue sky, do this in a dim lighting, although I think it can be done in bright setting just harder.  Then with the un-covered eye begin to stare blanky at the same spot  (faster if you unfocus your eyes too), you'll notice from whichever side your covered eye is, darkness slowly does begin to engulf it? exactly like when you do it in the dark.Why?

[John C Hagan III, MD, FACS, FAAO]

Because of the unequal amount of light entering each eye the photochemical systems are unbalanced for a short time and what you observe are the eyes catching  up and synchronizing. This assumes healthy eyes. If one eye is not healthy then of course they are not balanced and things will look different.

[Harveyclues]

VERY interesting, it is defiantly a hard topic to bring up in person, although a expert as yourself described it with ease, also cured my tiny panick. So I will leave on this, Thankyou very much John C Hagan, alot of respect for you and your help and work you do in life as well as take time to help people on MedHelp.

[Harveyclues]

Also, what do you mean by 'this assumes healthy eyes'', are you saying my question is a normal response, or are you simply stating that if this were to happen with both eyes open, then it would be wrong because it's not purposely caused?

[John C Hagan III, MD, FACS, FAAO]

It means, as with most posts here, that you have had a recent eye examination by an Eye MD or Optometrist hopefully to whom you mentioned this problem.

[Harveyclues]

I had a eye exam two months ago, didn't mention this though, I did ask a few friends and they could also reproduce it. I wasn't noticing it then because I never tried too.

[Harveyclues]

Would you class this as a 'problem' if everyone can do it, I'm confused now.

[John C Hagan III, MD, FACS, FAAO]

No I don't think its a problem

[Harveyclues]

Thankyou again, sorry I have trouble understanding replies sometimes, have a good day Hagan.

[John C Hagan III, MD, FACS, FAAO]

Day is done but I'll try and enjoy the evening.