Edge of Blur is an illusion

Between my computer engineering background, and the view I have of things without my glasses, I’m hyper aware that what I see on the computer screen is an illusion. Each of those curvy letters is really an array of blocks stacked up, red, green, and blue blocks are even separate, and you only think you’re seeing a white pixel. Ideally the distance between blocks is smaller than the smallest discernible difference the eye can see, but when you look at the “O”, and you think you have no blur, that’s just your brain filling in the missing bits of information to make that blocky display and speckled rods and ganglion compression into a smooth curve.

It’s less of an issue with print, but at some level it also is rendered in dots per inch, not smooth curves. Text starts at 300 dots per inch, can be much finer with laserjet. Ink runs slightly though, so the dots in a field tend to merge together and produce smoother edges. Cheap printing on color packaging though is sometimes still obviously dot matrix on the images.

Not saying it isn’t a useful measure. You do want to know when the amount of blur is more than your visual cortex can compensate for. Degrees of blur are real, but what we perceive as clarity isn’t necessarily directly related to the focus of the light falling on our retina.


I guess the edge of blur is the edge of additional distance-related blur.

Too small to resolve isn’t actually blur is it?

I think our eyes and visual cortex were designed to see the world in a certain way. More fine detail would just overload our ability to interpret the world we see.

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In theory light rays can carry a crazy dense information signal, we’d need brains the size of a house to process it all, and by the time the signal reached the back of our brain it would be too late to react to what’s happening in front… (This is why some reactions are handled by reflexes in ganglion instead of traveling all the way to the brain. Nerve signals take time to travel.)


Yeah, edge of blur is quite a fuzzy concept, if you pardon the pun.

I’ve been rather amazed at my brain’s ability to pick out detail (especially numbers/letters) from blur. But I don’t knock it, if it gets me another 0.25D of reduction, then that means I get to 20/20 that much sooner (even if I still have months to “settle” it in)

I’ve also struggled a bit at having an objective definition of “edge of blur”, What I do now is use the following criteria:

  1. My glasses are strong enough if I can make out details, edges, etc, even if there is “scatter” of non-focused light around it
  2. For computer work (I’m also a software engineer), I use the weakest glasses where I can sustain sharp-viewing detail over hours, without making seeing what I’m working on “too hard”.

To ensure I’m at the “edge” of blur, both of the above require seeing important details sharply, though I’m a fair bit off from the contrasty, knock-your-socks-off clarity. This means I’m:

  1. Using strong active focus (near my limit)
  2. My brain is sorting through the images it gets to process out details. How well it does that establishes how far “into blur” I am for me.

Since the whole angle of this myopia reversal is active focus, the above criteria is “good enough” for me. On the computer, I will up my prescription if I have to “work too hard” to understand clearly what I see while I work. If I have an easy time of it, I’ll drop my prescription. That way I always tune my view to maximize sustainable active focus during my hours in front of my computer, and auto adjust as my eyes fluctuate from day to day. I do similar parts with my other aspects of life.

As a result, I rarely measure centimeters anymore. Too much noise and variation, meaning I’d need some significant statistical analysis over time to show an improvement (and that improvement could simply be a changing sense of “good enough” on the blur levels).

With the above criteria, I can much more easily track progress. Stepping down with glasses and still being able to see well enough to comfortably work provides all the feedback I need :slight_smile: Though I’ll cross check by seeing if I’m leaning too close to the monitor, that tells me I’m dropping a bit too soon!


Yeah, I’ve so far been unsatisfied with trying to measure my distance to blur. It’s somewhat possible now that I have the -12 contacts, but there’s the distance to doubling before the distance to blur, and maybe I have some astigmatic blur in between… Consistently telling those points apart is hard.

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Fortunately, diopters are only available in multiples of .25 and Endmyopians tend to measure distance in cm rather than in mm, so in most cases, our tools of measurement are crude enough that the very small error in detecting the edge of blur can be dismissed.
In my case of high cyl/sph ratio, cm measurements from text have never worked for me because I can record the distance to directional blur but by the time I get to the distance to myopic blur, the text is already very messy and it’s incredibly hard to tell where it starts. So I’ve tried to set other landmarks for myself; I measure with the Snellen, with outdoors signs, and I plan to start using the wheel for cms so that I can measure the myopic blur specifically on the one clear axis.
So if your myopia is so high that even the screen resolution or the few mm of the brain smoothing the edges matter to your measurement, then you could find another way to give you feedback. Feedback does not have to be in cms once you know what you’re looking for, just notice the one thing that you can always come back to to see variations in your own eyesight.

Oh, a bare eyed cm measurement would have even bigger issues for me personally. Even if the edge of blur was a sharply defined line, a quarter diopter is a very small distance, and getting the ruler placed consistently and measuring something equivalent enough to the eye surface to get that mm accuracy…

No, this is just a discussion about objective reality vs perceived reality. All that really matters for what glasses you wear is perceived reality.

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Absolutely, and that goes a long way in demystifying corrective lenses. At one point in my life, I’ve seen reading glasses sold, not in grocery stores, but right on the street, over some cloth on the ground without even documenting the power associated with each lens. Customers would just come, try them on, see which one feels good and helps them read, pay for it and go.
Getting fit for glasses at the optometrist’s is still like buying a suit tailored for you. It’s expensive and very personalized, but still somewhat imprecise.

And then if we go down the rabbit hole of perceived reality, we can’t even see all the light spectrum, or hear all frequencies. I wonder how small the part of reality we perceive, and how accurate it even is.


I feel like this discussion is down to the path of philosophy and can we found our way back to “reality”? :rofl:

Perhaps I should do that instead of measuring on printed text, but I would have to tilt the wheel a little to get the right axis for any given day. A few more spokes on it would have been useful - having two equally dark lines is not. I would have to print out a clear, crisp wheel of the right size to put on my ruler as a sliding card, as this is the way I measure. I prefer the full wheel, as the continuation of the darkest line into the lower half confirms it nicely for me.

Found one with more spokes. I have already printed it out but not yet on a card. I can already see I have been deceiving myself on measurements to text. :frowning:

Now turned into a sliding card. There is still no one darkest line - three of them are equally dark. The ghost slides off at the same distance 19-20 cm but the dark lines stay darker than the others out to 28-29 cm, so not all that different from the text.
This is in not so good artificial light, as it is still early here, and I will measure again outside in good daylight.

Now measured in full daylight, and it does give me about the same results as my text card. The advantage is that I can also confirm any big change in axis in the left eye pretty quickly. At present it is still close to oblique mirror image astigmatism, which cancels out a fair bit in each eye with binocular vision.


Perhaps I can point you towards the tool I conceived:
Simple tool to characterise blur

The idea was that directional blur would manifest by closing up the gaps on the axis that appears darker, and myopic blur would then fill in the gaps on the orthogonal axis.

An interesting effect that I hadn’t anticipated is that my ghosting tends to make the white gaps appear to double.

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I see this only as I move out well past where I would judge directional blur. As things get fairly blurry the perpendicular white spaces become prominent.

@Ursa, nice find on the image.

The white gaps double way past the distance where the ghost image has already been with me for a while.On none of the lines are the white gaps actually filled in.

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Did you not say yourself that one should only believe what you see with your own eyes? That is perceived reality. In the context of our discussion, how the glasses feel to you and how well you see in them matters more than the math and the objective science of it.


What I think happens here is that the gaps appear to double once the blur from the two sides of the gap start to overlap, making a darker ghost.

It’s similar to the effect where you bring your fingers together so that they meet at the tips just in front of your face, while you are looking past them. The “floating finger” appears once both eyes agree that there’s “something” there.

Took me a while to see a Frankfurter, but not in the middle. It encroached on my left eye space. This is probably because my eyes are so unequal.

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I came across this interesting optical illusion that demonstrates the ability of the visual cortex to “clear blur” on its own.
It seems active focus is at least in part a visual “illusion” of sorts.

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i was just joking and you took it seriously :sweat_smile: and I total agree with you, how we feel matters definitely more than whatever machine told us how our vision “should be”.

The illusion is part of “Visual delay” which caused by residual image/color in our visual cortex, the technologies they made movie and TV shows are invented based on this effect. I think we all have this experience since childhood, but I only come across this term when I studied psychology last summer, which I find it quite fascinating to learn more details about it.

To be honest, some of the AF experiences do feel like related to the visual delay, the residual image/color sometimes counters/covers the blur area and made the object seem sharper. but I’m not sure all the AF effects are contributed by this, I’d rather believe it’s not. :sweat_smile:

neither less, since a lot of people got visual improvement (lager than what can be explained by illusion) here by practicing AF and establishing good habits, let’s just keep doing what we are doing and leave the root cause for experts to find out (including scientists, psychologists and philosophers :grinning:)