I was using glasses with the following parameters:
left eye: -2.50Dsph -0.75DCyl right eye: -2.50Dsph -0.75DCyl
Recently, the optometrist prescribed to me glasses with the following parameters:
left eye: -3.25Dsph +0.75DCyl right eye: -3.25Dsph +0.75DCyl
As she explained, these parameters are basically the same, i.e. -2.50 Dioptres with -0.75 cylinders denote the same lenses as -3.25 Dioptres with +0.75 Cylinders.
I don't really understand the physical meaning of this: as far as I recall, dioptres are related to the diffraction coefficient, and the curvature of the lenses. In other words, a lens should has exactly one value for dioptre.