How do positive and negative astigmatism differ?

January 6, 2012 6:36 AM

Why are there two different methods (positive and negative) for recording astigmatism?

I'm led to understand that surgeons typically write positive cylinder numbers, while optometrists typically write negative cylinder numbers, and that negative values can be converted to positive ones by adding sphere+cyl to get the new sphere, and inverting the sign of the cyl to get the new cyl.

(1) What, historically, led to this situation?

(2) There seems to be some sort of difference in Lasik between "positive cylinder programs" and "negative cylinder programs", which seems to suggest that there's some actual difference. E.g., What is this talking about?

posted by dmd to Health & Fitness (3 answers total)

For correctness/completeness: you also have to rotate the axis by 90 degrees.
posted by dmd at 6:55 AM on January 6, 2012

See the description on this link
posted by tehloki at 7:29 AM on January 6, 2012

and quoted, in case that wwebsite disappears from as on internet:


Some ophthalmologists and a few older optometrists write prescriptions in plus cyl form. Many years ago, refracting instruments and spectacle lenses were ground with the cylinder correction on the convex, front surface and hence the rationale for plus cy l notation. During the last 20 years or so, most refracting instruments and nearly all spectacle lenses have been ground with the cyl correction on the concave, rear surface. There are resulting subtle optical effects between these two lens manufacturing techniques and is more pronounced in higher powers. Optical purists would prefer that if your optical correction is measured in minus cyl, then your spectacle lenses should be ground in that manner and visa versa. Most optometrist consider plus cyl lens es archaic as they are difficult to find as stock lenses, although they can certainly be custom fabricated.

posted by tehloki at 7:31 AM on January 6, 2012