Question : How do I convert the power of my toric lens to spherical lens?
I have astigmatism in my left eye and my prescription is:
-4.00, -1.75, 180
How do I change that so I can get circle lenses (which aren’t available in toric)?
toric lens

Best answer:

Answer by yagman
What you are asking is how to calculate the spherical equivalent of a toric prescription. The way you do this is to take 1/2 the value of the astigmatism component and combine that with the spherical component. The problem is 1/2 of -1.75 is not a power that is available since all powers come in 1/4 diopter steps. So, it would be best to take 1/2 of -1.50 which is -.75. So now take the -4.00 and combine that with a -.75. This would be -4.75. So the best possible power based upon your prescription would be -4.75. I doubt, though, that you will be satisfied with the clarity of your vision wearing this power. I would recommend you stay with a toric lens.