Archived from groups: rec.photo.digital (More info?
> Optics is an unforgiving science. It turns out that for any (typical)
> optical system, one can only optimize for one particular
> camera-to-object relationship. Given an object distance, the lens
> lements can be subtly bent to optimize spherical aberations, comatic
> aberations, astigmetic aberations, field flatness, distortion, and
> last but not least Chromatic aberations. Once bent, the lens as a
> whole has been optimized for this lens-to-object relationship, and
> the lens will be less sharp anywhere else. Notice that the changes
> are occuring to the glass elements themselves, to to their spacing.
> A 'regular' camera lens is optimized for a focus point close to
> infinity. Optimized means that the aberations are made as well as the
> lens designer can do for that focal length (almost infinity).
> A 'macro' lens is optimized for lens to object distances close to the
> focal length of the lens, adjusting the shapes of the lens surfaces to
> optimize this different goal--sharp focus at very small distances.
> When a regular lens is used at very close focal distances (as a
> 'macro'), the shperical aberation can become unusefully larger than
> the 'regular' case. With a small sacrifice to coma and astigmatism, an
> interior lens (or lens group) can be moved a small amount to optimize
> the lens for a focal distance only a few focal lengths away from the
> front element of the lens itself. Done thusly, the combination lens is
> passable as a macro.
PS: I'll appreciate it when you leave enough of the post you are
responding to so I can understand your motives and references.