Hello everyone,
I would like to discuss reading at the edge of blur, which I guess means reading at the point where the letters are almost unreadable. Has anyone ever tried this vs. reading at the edge of focus like normal?
Hi Jansen,
Good question - a fairly sophisticated one. So I can tell you are putting a lot of thought into this. It might be good for you to first puzzle this out and write down your guess before you read my answer.
Let us distinguish three distances:
D1. The 'edge of focus' which we'll define as the furthest distance for myope (or closest for a hyperope) where a printed letter is completely in focus
D2. The 'edge of blur' which we'll define as the distance just beyond the edge of focus, where a slight blur in the letter can just be detected
D3. The 'edge of readability' which we'll define as the furthest distance where you can intelligibly recognize what the letter is.
Now D1 and D2 are going to be VERY close, almost exactly the same distance. If you are reading at D1, and you push the print slightly away less than an inch, you are immediately at D2. And if you are at D2 and get the tiniest distance closer, you are back at D1 again.
But D3 (which is what I think you are calling "the edge of blur", but is really beyond that) might be a fair distance away, perhaps even several inches (for small letters) or several feet (for large letters). And D3 will also depend on your familiarity with the letters and the language. You can "guess" a blurry word like "eye" because you know the English language, whereas if it is a word in a different language you might not be able to read it. So D3 depends on the size of the letters and your ability to recognize the letters and words. D3 depends upon your brain, not your eyes. You might be able to read a very blurry version of the word "eye" and guess it, whereas a non-English speaker might guess the wrong letters.
To get the benefit of plus lenses, your eyes must be working "automatically" to try to focus, so they must be able to detect
the direction of defocus, that is, whether the blur is caused by being too close or too far. Your eyes detect this defocus right within the retina -- the back of your eye. This detection process does not involve your higher brain functions that are involved in word recognition or trying to guess what a word means. So you must allow the eye itself to detect the defocus.
Therefore, ideally your focal distance should be at D2 -- the edge of blur.
This should be the slightest blur detectable, and it is even OK to move back and forth between D1 and D2 to keep "testing" this distance. If you go beyond D2 to D3 (the edge of readability), you are now at a place where your brain is doing the work of guessing, but the focus detection system in your retina is no longer able to detect the direction of defocus. It's not just that the eye muscles and lens "give up" trying to focus, its also that you lose any stimulus for change of the structural tissue around the focal plane at the back of your eye (the retina). So your eye gets no information about how it needs to change.
I hope this explanation makes sense and is not too complex. Let me know if it does not seem clear to you and I'll try to do better. But it is an excellent question you asked, because to answer the question you really have to understand what is happening in the eye.
Todd