Strange appearance of point-source without glasses

I have noticed something for quite a while that I just cannot explain. I am farsighted with about 2.0 diopters of distance correction needed in my glasses. I am 72 years old, so I have very little inherent focusing power. Therefore when I wake up in the middle of the night without my glasses on and look at the tiny green LED status light in our smoke detector on the ceiling about 20 feet away, I expect it to appear fuzzy. But it does not exactly look fuzzy they way it would appear in a camera that is out of focus. Instead the tiny point source of light spreads out into a circular constellation of points of light. What is more, the constellation of points is different in the right eye and the left eye. Also, every time I blink, the constellation is rearranged slightly. My left eye needs more correction than my right eye, and sure enough, the constellation of points seen in the left eye is a little more spread out than the one as seen in the right eye. And if I squint, the constellation narrows in both eyes (as expected). But it gets even weirder. If I have only one eye open and then slowly move my finger in front of my eye, the constellation gradually gets obscured, but from the opposite side from where the finger is coming. This happens if the finger is right close to my eye and it also happens if the finger is at arm's length from the eye. The cutting off of the points in the constellation is sharp - not fuzzy. It is almost like a curtain is being drawn across an image.

I know a little about cameras and photography, and this is like nothing I have ever seen in cameras when they are out of focus. Does anybody have any idea what is going on? My eyes are fine with glasses on, and I have almost no astigmatism.

 

Hello Numbers Guy
I have studied vision in psychology. I see what you see, (see image) but I have myopia. It's nothing unusual. However when you block the apparent point lights with your finger they should logically disappear one by one according to the true position of your finger, rather than from the opposite side. So that part of your experience seems very strange. You should be aware that the eye is not a true analogy with a camera. An out of focus point image on a cinema screen, for example, will appear as an expanded bubble. However, when the eye casts light to the retina it is a totally different phenomenon. The light sensitive cells each respond to the light individually. When you focus, muscles in your eye change the focal point so that only a few central cells receive the light. It's very interesting to think about. I have mentioned this to several optometrists over the years and each one seemed at a loss to interpret my blur. Perhaps the camera analogy is to blame!

 

Thank you for your response. But I have figured out the reason for the "opposite side" disappearance, and it has to do with the difference between your myopia and my hyperopia. The normal operation of the retina is to receive an inverted image of what is in front of the eye, as show in the image below. The letter "A" in front of the eye is cast inverted on the retina, and the brain interprets the image correctly by associating "down" on the retina with "up" in the image. Now look at individual rays of light. The rays entering the top of the lens are refracted and hit the retina too low for myopia and too high for hyperopia. So whatever obstructs the rays at the top of the lens will affect what looks like the top of the image to you, but the bottom of the image for me. I have confirmed this with a friend who is myopic like you. She also saw the constellation, and, like you, reported that the portion of the constellation that is obstructed by her finger is on the same side as the obstruction, while to me it appears to affect the opposite side. It is just geometry. But I still don't know why anyone sees a constellation instead of a blur anyway. If you want to try to see what I see, try to borrow some glasses for myopia, but at about 2 diopters stronger than your correct prescription. Wearing such glasses will give you the hyperopia experience. Or if you have two pairs of glasses with your correct prescription, try wearing them both together. That will do it too.

 

Thanks for your explanation about hyperopia, it makes perfect sense.

I already did as you suggested i.e. overlay my spectacles with another pair. My wife and I saw the LED light as a constellation. That aside, I tried simultaneously looking at the LED light through one eye without glasses and with the other eye with glasses. The point source was the same size in the corrected view as with the uncorrected view. There was no expansion of the point source like an out of focus camera lens. But each constellation point was much less bright compared to the corrected view. So why is this happening? This is the interesting part.

Consider an old-fashioned slide show. Let the slide be a sharp picture of the moon at night. If the light coming from the moon picture does not converge at a point, the image is blurred. That is to say, the light from the edges of the moon, and from the body of the moon, are dispersed over a greater spatial area. The retina, unlike a projection screen, is an arranged pattern of neurons. If you tried to explain this to a blind person you could suggest thinking of these individual neurons as individual observers. Instead of light, let each observer receive a sound signal that has been dispersed by the air. Each observer receives a different version of that sound, yet the resolved quality of the sound is unchanged. Only the loudness is slightly lower. Similarly, each neuron responds to a different segment of the incident light that strikes it. The image is not ‘blurred’ but it is lower in intensity.

Interestingly, when I take off my glasses and look at the moon at night, I see a pattern of many slightly less luminous moons. Now, under lower contrast conditions of daylight there is so much interference going on from the light of other reflected objects and bright background that my brain registers objects as a blur. In this respect the retina is like a projection screen because the end result is the same: a blur. Of course, this similarity is superficial as our experiments have shown.

I did a random internet search and found an article in the Journal of Vision, May 2012 “Shedding light on night myopia”. Due to the aberration of light, stars can look like little constellations even for corrected vision. It’s free if you care to look it up. It proves we are not abnormal!

 

Another factor to consider regarding high light levels and low light levels is that in low light levels, the iris (which is comparable to the iris in a camera) opens all the way up, and thus uses the whole lens. Therefore imperfections in focus are more exaggerated (just like they would be in a camera with the lens wide open). But in high light levels, the iris closes down to just a small hole. This has the effect of sharpening the focus and raising the depth of field, so that essentially everything is in focus. This is the principle of a pin-hole camera, which can project a reasonably sharp image on a sheet of paper using no lens at all - just a pin-hole. So when focus is less critical, everything looks sharper.