Meanwhile I have a prescription with parameters in order (left to right, in the grid) like this: “sphere, power, cyl, axis, …” and the “power” column is filled in but “sphere” column is not filled in.
Can anybody explain what’s happening? Is this “power” column actually same thing as “sphere” parameter that is explained online? If so, what does the empty “sphere” column represent in this weird prescription?
I would presume that the “sphere” parameter is something like a radius of curvature. That wouldn’t be all that useful, though, since spectacles can be made from different materials with different indices of refraction. Power (which I think is basically the reciprocal of the focal length) would be more universally relevant.
Some research indicates that sphere and power are probably synonymous. If the power column is something like -1.75 or +2.25, then that’s probably what you need.
Apparently spherical and cylindrical powers do different things (cylindrical is for astigmatism). If you just have the one power number, it’s probably just spherical.
I’ve got degrees in Optics, but the ways of Optometrists and eyeglasses are frequently strange to me.
This page on prescriptions doesn’t have a “power” column, and neither do my own eyeglass prescriptions:
“SPH” is the spherical power, or the base power. “CYL” is the cylindrical power added to that if you’re astigmatic, and “AXIS” tells the direction that cylindrical power lies along. Since “SPH” is already a power, it would seem redundant to have another “Power” column.