The lens used in this Magnifier:
Aberrations such as coma, distortion, and chromatic aberration almost exactly cancel out also due to the symmetry.
Biconvex lenses are also called convexo-convex lenses and are mainly made out of a special quality glass. Different types of biconvex-lenses exist such as the planconvex-lens. The most common biconvex lens is the symmetric-convex lens as used in this magnifier.
The biconvex lens is manufactured with identical convex surfaces each side of the lens.The focal lenght is equal to the radius of the curvature. Resulting from its perfect symmetry it produce images close to 1 and up to a magnification of 8x. Biconvex lenses have positive focal lengths and form both real and virtual images.