That’s entirely possible. Try giving an emmetrope minus lenses and you will do just that. The mechanism is that too strong of a minus lens focuses light too far behind the retina causing you “blur while overcorrected.” The same applies to everyone. For example, it goes like this: if let’s say you are overcorrected and you see 20/30 with -5, you would reduce to -4.5 and see 20/16, then reduce to -4 and see 20/30 again.
My dad is an emmetrope with perfect distance vision and I gave him -0.75 for a quick second to try out against the Snellen, and he said he barely saw the third line from the top on the chart with them (which is 20/40 line on the chart that I’m using). This roughly corresponds with what a -0.75 myope would see.
Yes, he is. About +1 or +2. However, presbyopic or not does not matter. As far as I understand, presbyopes have no problem keeping their lens flat, so in essence, when they are looking into the distance, there is no difference in how the light refracts and hits the retina when compared to people without presbyopia. When one puts on a “too strong” minus lens, doesn’t matter if he is presbyopic or not, the intraocular lens would need to flatten even more than possible to make light rays focus on the retina instead of behind it. In neither group this is possible, because the lens is maximally flat when looking in the distance in both emmetropes with presbyopia and emmetropes without presbyopia. Consequently, both should have the same distance visual acuity with the same lenses. I hope my understanding is correct, maybe someone with more knowledge in optics can give their two cents.
I don’t know how often this happens. I don’t think it happens too often because measuring eyesight and picking the right glasses is the bread and butter of optometrists and if they overprescribe to the point where you see less than ideal it means that they can’t even do the most basic function an optometrist does.
I’d like to discuss this a bit more. It may not be a problem that presbyopes have little issue keeping their lens flat for distance. What may be a bigger issue is that non-presbyopes may have difficulty not keeping their lens flat for distance viewing, especially through minus lenses…
My understanding is that for many people, achieving 20/15 takes an extra -0.25D to -0.50D in lens power beyond what would be considered passable 20/20 (in other words, normalized/comfortable/just barely full correction). The effect of this is the same as which results in emmetropic people with a slightly positive refractive state (+0.25D to +0.50D) often having slightly better distance vision than emmetropic people with an exactly zero or mildly negative refractive state (since emmetropia goes from -0.50 to +0.50). Of course, the tradeoff is likely worse close vision for people on the positive end.
Similar effects can be generated in those emmetropes with a 0.00 or even slightly positive refractive refractive states. It’s a well-known phenomenon that giving these people minus can sharpen the image in the same way as with a myopic person who is overcorrected (assuming they don’t yet have presbyopia and have enough accommodative amplitude to focus through the minus).
The effect of this is a boost in contrast and smaller image.
This is why good optometrists ask people if the image is truly better, or just smaller and more contrasty, when adding minus.
I think trying to understand why it takes a slight overminusing for some people to get 20/15 would help us to better understand the ideas at play in my original question. One possible explanation I’ve heard as to why minus lenses sharpen things up like this has to do with aberrations in the eye. The idea is that accommodation neutralizes certain natural aberrations and enhances the image. There may be other reasons, too. (Physics experts, please feel free to comment.)
So, the situation I was thinking of, is a person capable of focusing through the lens, but for some reason focusing past the stimulus, resulting in slight myopic blur even when overcorrected. Refractionists talk about high myopes who “eat minus”, for example…they keep wanting more and more power beyond what should be the end point. This would also potentially explain why some people end up overcorrected by a diopter.
Or, maybe it’s much simpler and they just aren’t recognizing the slight hyperopic blur that you mention.
And why do minus lenses that are overcorrecting result in worsening myopia, but a natural refractive state that is positive in adults tends not to (unless they do excessive close work)? The reason may have to do the power gradient in the minus lens, which @Varakari has recently brought up.
Another thing, most exam rooms are 12 to 20 feet long, so keeping the lens flat may be harder than at actual optical infinity due to the walls. Additionally, the low light conditions often used may induce dark-focus myopia (where the eye habitually reverts to a slightly negative accommodative state without sufficient visual contrast).
I feel like this is a topic we can delve more into, and really analyze what might happen in various situations.
With my prescribed -6.5, it was hard to get my vision clear.
Since starting EndMyopia about 2 months ago, my correction has reduced to -6 and I see clearer than I have in a long time. So much so that I believe I am due for a new normalized.