It’s been said that one can add 0.25 to 0.50 in SPH to compensate for reductions on CYL. Will doing so cause axial elongation?
Hyperopic defocus causes axial elongation. If you are wearing correct normalized and differentials this small added spherical shouldn’t be enough to cause that. Basically, if you aren’t over-corrected, you should be fine.
Thanks Laurens I guess that answers my question. That it’s BAD to increase SPH to compensate for reducing CYL.
No it does not, you use differentials for close-up so increasing SPH doesn’t cause hyperopic defocus in any way. Do what you like and go take care of it the way you want to, just know that sticking to EM principles here doesn’t cause the problem you’re worried about.
Yes, what Laurens said. Increasing SPH would not be very recommendable, in any scenario, if you end up with way more power than you need for the distance you plan to use it for. That generates hyperopic defocus which causes axial elongation and increased myopia. In the case of differentials you are already undercorrected, so as long as you have in mind active focus and blur challenge, there is no problem in doing so.
Compensating SPH for CYL in a reduction is just another (smoother) approach of reducing CYL. Doing trial and error you may find the way of reducing you find yourself most comfortable with.
Apparently I’m not being clear. Here’s an example to illustrate my concern. Let’s say, hypothetically, that a person has -4.00 SPH and -1.00 CYL at 90 axis. What that means is that on the 90 axis they can see clearly with -4.00 SPH. But on the 180 axis they need -1.00 CYL additional correction.
So then, if that person reduces the CYL to -0.50 and increases the SPH to -4.25 to compensate, then that means that the 90 axis is now OVER CORRECTED by 0.25 CYL. (or more if they drop the CYL to zero and increase the SPH to -4.50)
Do this makes sense now? I’m concerned that over correcting one axis could lead to axial elongation. And maybe even elongation on just that one axis. And possibly make things worse? See what I mean?
BTW I totally get the point about differentials and not being too close with a too high power. My main point is that the eye “perscription” is now unbalanced. Under corrected on one axis, and over corrected on the other regardless of the distance to that which you are viewing.
My guess is that you are right. If the -4.00 SPH CYL -1.00 is their full prescription, then they would then be over prescribed by -0.25.
But if this is actually their normalized prescription then the SPH -4.00 is at least 0.25 below their full prescription. Shifting -0.25 over from cylinder would make it SPH -4.25 CYL -0.50 and that wouldn’t cause elongation because it would put them at or below their full prescription.
Yes totally I agree. You know lots about this subject. Thank you. A question: Does too strong for differentials cause elongation too? Referring specifically to looking in distance, not up close.
Have a good look at this amazing post: How to lens-induce your eyes
Now consider your question again. Differentials, even stronger ones, you look in the distance with them, would you get that focal point on your retina? No way José. It ends up before the retina, hyperopic defocus is the focal point getting put behind the retina. This is possible with normalized or full correction lenses and then working with those on something up close.
If you get the physiology this all makes perfect sense. Proper differentials and looking at something distant will only give you myopic defocus so there is nothing to worry about.
Great, thanks. I mis-stated “differentials” when I meant “normalized”. So then you’ve confirmed for me that too strong for distance is bad. That’s what Jake says too. So then what happens when it’s too strong only on one axis? I’m wondering if it’s good or bad. Here’s what’s established: Too strong SPH for distance causes elongation. But does too strong only on one axis cause elongation too? And if so, does it force the astigmatism to elongate to compensate in a good way by making the eye elongate only on that axis? Or does that make the astig worse?
The eye is able to register if the focal plane is too far in the back of the eye. If this is something persistent and present all the time the eye will “conclude” that it’s too short and it needs to elongate. If it’s just on one axis it could cause that too. If it happens enough and if that’s the angle that it utilized more often than not it can be the case. However, sticking to EM teachings, you swap out 0.5 dpt of cyl. for an extra 0.25 of sph. That means that you “under-correct” that axis by 0.5 dpt already. It’s pretty hard to to keep an over-corrected astigmatism axis going if you follow EM principles properly.
Don’t overthink this too much, make a couple of successful reductions and then see if you can swap out some of that cylinder correction, EM is all about getting the eye to revert back to it’s natural shape. Everything we do is geared towards that, astigmatism reductions are no exception.
Thank you Laurens! I over think everything. I actually get paid for doing that in my marketing firm! Anyway the reason I started the OP was because I was feeling strain in one eye where I was over-correcting a little in SPH because I reduced a little in CYL. So that got me to thinking. I’ve since reduced the over correction down to a point where I’m not feeling strain. Overall EM is working good for me. My CYL is now at zero for differentials. Still have some correction for normalized.