Why do we need less cyl correction in differential than normalized?

Astigmatism means that the part of our eye is more myopic than our whole myopic eye. for example, if our eyes are -3sph and -1.25cyl it means that there are parts of our eyes that are more myopic -1.25D than -3D.

but why do we need less cyl correction in differentials?

Doesn’t looking at close range with a diff with less cyl correction still leave a directional blur?

or reducing the cyl in the differential is only used if we want to improve astigmatism?

Yes, but less of it. So the same principle applies as for spherical correction - only use as much as you need. This is a bit harder to calculate for cylinder, and for most of us it involves a bit of guesswork. A friendly opto who is willing to measure you for computer glasses could help you out here - ask for as little spherical and cylinder correction as you can get away with at that distance.

The actual amount of directional blur is proportional to the distance. So yes, there will be directional blur in close-up, but it won’t be noticable.

That does depend on the degree of astigmatism - it is definitely still noticeable in my left eye at screen distance, but compared to what astigmatism in that eye does to the moon, it is nothing. :full_moon: :full_moon: :full_moon: (three of them, but overlapping in a circular shape).
So for high astigmatism, it is a bit of guesswork deciding how much of it you can drop in diffs.

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can it disturb our spherical gain?
because our eyes work in spherical and directional blur at the same time??

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I read on the blog that we don’t need cyl correction on the differential if our astigmatism is less than one diopter. but does that apply if we want to do a cyl reduction or also for spherical reduction?

The goal is that you don’t notice it. If you notice it, then yes you are doing and equalizing step and then it may disturb your improvements.

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why don’t we keep using full cyl correction in differential?
For example
Normalized: -4.0 -0.5 180(sph reduction only, keep Cyl)

I reduce the differential by -1.5sph diopter and eliminate cyl correction entirely because my cyl correction is less than 1 diopter

So my Differential is:
-2.5Dsph without Cyl

isn’t using this differential, I will get SPH blur at 66.66 cm and directional blur at 50cm ???
because some parts of my eye are more myopic 0.5D than 66.66 cm SPH blur?

Then, at what distance do I keep doing active focus? is it at a distance of 66.66cm or 50cm? if I do it at a distance of 50 cm isn’t that the same as me doing active focus for astigmatism (not for myopia)
@halmadavid
@Ursa

You seem to think that active focus can be done only at the very edge of blur. I can do active focus at all distances. As long as I can make the image a little bit clearer - not necessarily perfectly clear, I consider it as AF.
Of course if you spend a lot of time at a screen, you might want to keep a little cylinder. It really is up to you to experiment.

@Ursa
if so why don’t we use full cyl correction for that differential?
If on that differential we use full cyl correction, we will do active focus at a distance of 66.66 cm in SPH blur without any interference from directional blur and it is also easy for our visual cortex.

If that is the way you want to go, do so. But at some stage you will want to tackle the astigmatism as well, and as full cylinder correction is believed to be the cause of increasing astigmatism, you may want to keep this in mind. I didn’t start life as a myope with -2D of cylinder in my left eye. It got worse over the years because the little I had at the start was fully corrected for, got worse, was again fully corrected for, etc., etc. @Kem can tell you all about this.

@Ursa
Does that mean that in the differential we can do active focus on SPH blur and Cyl blur simultaneously?

Theoretically I cannot answer that, as we have no idea if and how active focus works to reduce astigmatism. I suspect that the visual cortex can learn to ignore some astigmatic ghosting. The differentials are mainly used to reduce overcorrection at screen distance, and my feeling is that AF is more useful at further distances, because it is there that our myopia lets us down. I have no personal experience with differentials as I never used any. I just slowly pushed my screen further and further away, and it worked. But I paid just as much attention to AF at far distances, first the 6 m Snellen and then further and further distances.
If Jake says that cylinder can be reduced for differentials, I assume he is basing this on the experience of many people in the years since he has been doing this.

why does astigmatism increase if we use a differential with full cyl whereas if we use full cyl on normalized our astigmatism doesn’t increase ??

I was talking about how we got high astigmatism in the past. I have no proof that astigmatism will increase from now on, but it seems reasonable to assume it does. The reason for not tackling cylinder in your norms at the same time as reducing spherical correction is to not overload the visual cortex with too many changes.

Either you are asking all these questions for the sake of asking questions, or you have not read enough on the blog and the forum. End of conversation, as far as I am concerned. No hard feelings, I hope. :slightly_smiling_face:

I assume the amount of error in terms of light rays being focused is a function of distance and there is also a limit how much error in focus your visual cortex can cope with. Hence less error at lower distance so you can do with less cylinder

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Do you mean that our visual cortex can fix SPH blur and cyl blur easily at close up distance? So it doesn’t matter if we do active focus on spherical blur and cyl blur at the same time using differential?

@Lajos How many cyls do you reduce for your differential from normalized?

nope. I mean that if you think about light rays, the longer path they travel the more distance they have to diverge. In case of astigmatism, your eye bends different light at different meridians differently so at a greater distance there might be more difference between different paths of light rays maybe causing more cyl to be needed for distance.
but it’s just my theory not any proven fact.

about 0.5 D for me, but that’s just personal preference

is it for spherical reduction or for cyl reduction?

My optometrist - who admittedly always measures to provide 20/20 vision for driving at dusk or at night, i.e. aims for perfect distance vision in worst lights after a day spent in close-up and being tired - measured 0.5cyl on one of my eyes, and said that should be added to my glasses, but if I wear contact lenses I can simply add half of it to the diopter measurement and not have cyl and that will give me the same acuity… as said this is from an optometrist aiming for perfect vision. So I guess a little cyl doesn’t really matter…