B The focal length of the eye and image inversion

[anuttarasammyak]

After my surgery this year, gas remained in my eye for a while. The light air bubbles appeared to sink to the bottom, and I realized that the brain was processing the information to invert the up/down/left/right image transferred to the retina.

I have a question about optics and ophthalmology. Does the inversion of the image transferred to the retina depend on the position of the intraocular focal point of the lens of the eye? For example, in people with farsightedness, the focal point is behind the retina, so does an uninverted image transfer to the retina? Incidentally, I am nearsighted.

 

[Ibix]

No, the image is inverted for all of us. Draw a standard ray diagram for a pair of thin lenses with a couple of off-axis points in the same object plane. Now consider shifting the screen backwards and forwards through the image plane. You'll induce blur in each point, but the points won't swap position.

If you look at a laser dot on a wall you will see laser speckle, and if you move your head you will see the speckle pattern flow. I am told that whether it flows in the same direction or the opposite direction to your head movement does depend on long- or short-sightedness. I have not had the opportunity to test that claim.

 

[anuttarasammyak]

Thank you @Ibix. I was caught up in a false association between focus and inversion.

 

[hutchphd]

I have not had the opportunity to test that claim.

In fact I have and it is absolutely true. I am farsighted and have reading eyeglasses that make me effectively nearsighted. The movement of the speckle (from a laser pointed on a matt surface wall at distance 3m) is easy to see and changes direction when glasses are removed. I know of no practical utilizations of this effect but intend to play with it soon and do some research. Will append any interesting findings here.

 

[kuruman]

And then there is the Erismann experiment.

 

[DaveC426913]

And then there is the Erismann experiment.

The only time I encounter this organically is when trimming my "back burns" in the mirror. If I keep at it long enough, eventually I can coordinate my movements.

In fact I have and it is absolutely true. I am farsighted and have reading eyeglasses that make me effectively nearsighted. The movement of the speckle (from a laser pointed on a matt surface wall at distance 3m) is easy to see and changes direction when glasses are removed. I know of no practical utilizations of this effect but intend to play with it soon and do some research. Will append any interesting findings here.

Well now I have to try this.

 

[A.T.]

After my surgery this year, gas remained in my eye for a while. The light air bubbles appeared to sink to the bottom, and I realized that the brain was processing the information to invert the up/down/left/right image transferred to the retina.

Note that you only see objects within your eye sharply, if they are close to the retina. Otherwise they are blurred across a large part of the image. So there is bias to which objects you even notice. And the visual location of those object is inverted, because of how the brain interprets the signals from different locations on the retina.

 

[mjc123]

The only time I encounter this organically is when trimming my "back burns" in the mirror. If I keep at it long enough, eventually I can coordinate my movements.

I can coordinate the movements of the right hand, but not (or only with difficulty) the left. Is this common?

 

[DaveC426913]

I can coordinate the movements of the right hand, but not (or only with difficulty) the left. Is this common?

Well, presumably it's affected by your handedness. I'm a leftie and I can't even operate scissors right-handed without a mirror.

 

[kuruman]

Well, presumably it's affected by your handedness. I'm a leftie and I can't even operate scissors right-handed without a mirror.

I always get disoriented when I see myself on a store's surveillance monitor because I instinctively interpret what I see as my mirror image. I guess the equivalent would be for you to try operating scissors while looking at a live selfie video.

 

[sophiecentaur]

I always get disoriented when I see myself on a store's surveillance monitor because I instinctively interpret what I see as my mirror image.

On a Zoom call with several others, you may notice that your image is arranged to be the familiar mirror-inverted but everyone else is 'right way round'. (And, of course they all see you the right way round.) We accept those things without thinking but, when talking to family kids on FaceTime , if you're trying to be smart and move about up close to the screen, it's not obvious where to put your face so that you are 'nose-to nose'. Such is the interesting life of a grandad!!

 

[sophiecentaur]

The effect of lateral invertion of the image in a mirror is easily coped with by the brain. We all use a rear view mirror in a car whilst looking at the traffic ahead and (I hope!!) cope with that fine. We don't give it a thought.
Moving a thin wire up and down very near the pupil shows it moving in the 'opposite' direction and floaters usually appear to float upwards when they are in fact settling to the bottom of the aqueous humour.

Far too much is (imo) made of the way the brain 'turns the image up the right way'. It nothing as trivial as that. It's all to do with mapping the real world with a pattern of light on the retina. It's not a problem to learn and establish what's really out there when using a mirror or fancy glasses.

 

[sophiecentaur]

@sophiecentaur

PS: People with very restricted or no vision still build up a similar image of their world. Hard to appreciate that when your established ideas are based on what you see and the way it 'obviously' relates to our input signals.
That comment is based on, for instance, the way we may have never seen inside our mouth but we (at least I) have a pictorial image of what it's like in there. Just think how surprisingly small an extracted tooth or crown looks when we actually can see it.