Why not reduce only a quarter diopter cyl in the differential?

In the differential, we are allowed to eliminate cyl correction if it is less than 1 diopter. But why are we not advised to reduce only a quarter diopter? if we reduce by 1D isn’t that too much?

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I think that Jake explains that when we eliminate the cyl correction if it is less than 1 diopter, we also adjust by substituting a half diopter in sph for the full diopter of cyl.

So, if my differentials would otherwise be at a -5.50 sph -1.00 cyl, I’d add back .50 to the sph to account for the drop in cyl therefore giving me a -6.00 sph and zero cyl. I think this makes the adjustment easier. It is working well for me.

If someone else can confirm that I’m explaining correctly & clearly, I’d appreciate it!

EDITED TO ADD: Okay, I double-checked and re: differentials and astigmatism, Jake recommends this: “You usually don’t want to have astigmatism correction for your close-up lenses. If your astigmatism is less than -1.00, it’s usually worth eliminating completely. Substitute half a diopter in spherical (just regular myopia diopter) for a full diopter of astigmatism.” Hope this helps you, Stevan!


No DSAs, but can I e.g. do:

N: -8.5/-3.5x180, reduced sph
D: -6.5/-2.5(-3)x180, sph is not increased


I double-checked and added Jake’s recommendation re: reducing cylinder on differentials if you have a -1.0 cyl or less to my note above - Thanks!

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Usually / often, certainly not a blanket recommendation. :slight_smile:


Thanks for the explanation

Sorry, meant to put that in there! Thanks for taking a look, Jake! :+1: