How to stagger reductions and Differential PD?

How do we stagger reductions between normalised and differential? (I understand we shouldn’t reduce both at the same time as that would be too much blur challenge).

What should PD be for differentials? Is it 3mm below regular PD? What if a former differential can become a future normalised yet the PD is weaker? Wouldn’t that mean we can’t use the old differential as a new normalised?

How do people go about bulk ordering corrections if they have astigmatism? My situation was simple. I bought 8 pairs of glasses ranging from -2 to -0.25 with no cylinder but if I had 1 diopter of cylinder, how would I choose what my future corrections would be? Would I have to buy 16 pairs i.e. -2 half a diopter of cylinder and -2 no cylinder, -1.75 half a diopter of cylinder and -1.75 no cylinder etc.?

Consider yourself lucky that you have no astigmatism. :smile: This is not the way it works. Once you have successfully removed the need for a cylinder correction, it should be gone for good.

I mean if a person did have cylinder. I’m asking for a friend. I don’t have cylinder but they do, so how would they go about reductions?

There is lots of information on the website and the forum, but you, or they, would have to do a bit of searching to put it all together. There is a whole topic section on astigmatism.

The very barest of outlines - you reduce cylinder correction a quarter diopter at a time, in between spherical reductions and usually not at the same time. It is difficult to know when one is ready for a reduction in cylinder, and it is not very likely that this will follow a clear pattern, so don’t buy glasses too far ahead of time. I did so, and regrettted it.
Don’t take this as adequate information - it is not, and please research astigmatism carefully before you give anyone any advice or suggestions.


Glasses with cyl come with 3 values: SPH, cyl and axis.
SPH is the main diopter, cyl is the added diopter at a certain angle only, and axis is the angle, so that stays the same (the axis never “reduces”).

The walk down to zero correction can’t be planned ahead, but basically there are 3 options.

  • Keep the SPH and cut the cyl completely
    Works best if your cyl is only 0.25 or 0.5, may work up to 1, especially if the cyl was overcorrected, too.
    Above 1 it most probably causes serious challenge to the brain, so better choose from the other 2 options.

  • After a few SPH reductions (when keeping the cyl unchanged, both strength and axis), once you feel you have some practice with reductions, keep the SPH and reduce the cyl by 0.5 (or maybe just by 0.25). Then do SPH reductions again, and when ready, reduce from the cyl again. This is the most standard method.

  • Increase SPH by half of the cyl dropped. This means you are not really reducing the overall strength, just on the sph - cyl difference, but you are simplifying the complexity of the glasses. If the cyl is high, don’t drop it in one step.

The thing with cyl is that your brain has a much more active role with composing the correct image from the eyes’ inputs. So it will need time and patience to let the brain adjust instead of the eye corrected only at a certain angle. It can easily take 3 times longer than a simple SPH reduction. Be prepared to tolerate failure, you may be stepping back and forth before you can really drop from cyl.

As for normalised and differentials, the most common is to keep the 2 aligned, so yes, you will not be able to use the early differentials as normalised later. This is why some people drop all cyl in one step at the beginning and walk the walk with simple SPH only.