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I understand that contact lenses are available which can correct a prescription with both sphere and cylinder components, however these are only available for large-ish values of cylinder (depending on manufacturer) and are also very expensive.

If someone has a relatively small cylinder, say only 0.50 diopters, I have found multiple references (here, here and this other answer) that say to add half of the cylinder to the sphere to get the best spherical approximation.

This is also exactly what I would naively expect based on my physics degree but with no particular understanding of optometry. However I was told today by an optician that he would advise a patient to order contact lenses just using the same sphere as on the glasses prescription, and disregard a cylinder of 0.50. He explained that it would be worse to have both meridians slightly out than one correct and one further off.

Are there any regulatory standards or documented references that give instructions what is best in this case? Do different companies have their own procedures or is it left up to each professional to advise their patient as they see best? (no pun intended).

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  • Geez, that was a mistake, sorry about that. But could you apply the other proper edits? Mar 2, 2022 at 22:40
  • I've added the tag. The other stylistic things are not worthwhile, given that this is a historic post which no one is interested in. (or since you appear to have a victorian position on grammar, "in which no one is interested" ;-).
    – Tom V
    Mar 3, 2022 at 12:51

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