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@RomeoNinov the ratio of the image distance to the object distance is defined as the magnification: \$M = i/o\$. Of course, that depends on \$f\$ as well: \$M = (i-f)/f\$.
Alan, that's the thin lens formula. The lens maker's formula uses the refractive index and the surface powers (i.e., reciprocal surface radii) of the lens.
