Precision glass moulding for a relatively small volume of material.” “As a lens changes to negative in power biconcave, plano-concave, and meniscus where the concave side is stronger, an alternative preform shape, plano-plano, is required for the molding process. […] Relative to a formed preform an increase in cost is observed for the manufacturing of this type of preform.” “When the geometry of a lens extends beyond the volume range of a formed ball preform, a ground and polished ball preform is required. Used for lenses with positive power: biconvex, plano-convex, and meniscus: where the convex side is stronger, this geometry allows for
Lens (optics) real object at the location of that virtual image. Likewise, it appears to a subsequent lens as if it were an object at that location, so that second lens could again focus that light into a real image, "S" then being measured from the virtual image location behind the first lens to the second lens. This is exactly what the eye does when looking through a magnifying glass. The magnifying glass creates a (magnified) virtual image behind the magnifying glass, but those rays are then re-imaged by the lens of the eye to create a "real image" on the retina.