Any optometrists on here that can help with eye prescription question please?

[Cumberlover76]

I have a glasses prescription from Boots and the cylinder value for my right eye is -5.00. I want to get some prescription sunglasses online, as they are much cheaper, but they only do up to -4.00 cylinder. Can I just adjust the cylinder measurement to -4.00? Or would the other values need adjusting too? I'd like to avoid having to go and get another eye test/prescription if possible.

Full prescription: R- Sp +0.75, Cy -5.00, Ax 29; L- Sp +0.25, Cy -1.00, Ax 117

Thanks

Not an optom but that’s quite a high astigmatism. When my contact lenses prescription was updated, & the cyl changed only slightly, it made things clearer so I imagine things would be blurry/out of focus if you ordered glasses with that big a difference. Would they even make glasses up that aren’t actually your prescription? It would leave them open to all sorts of problems if you drove & had an accident or stepped in front of a car.

i've got kerataconus in that eye which means even with glasses it's pretty blurry just through that eye, the cornea is pretty rounded as opposed to the eye itself. In the past an optometrist did me a less strong prescription so i could purchase online, but i was wondering if it is just a case of dropping the Cy value or if the others need to be adjusted. I can go back and ask for them to do me a prescription taking this into account but wasn't sure it it was as easy as just reducing 1 value.