Originally Posted by
John Morton
There is another consideration: true macro lenses are 'flat field' lenses. That means, for instance, a sheet of paper will be shown in perfect focus from corner to corner by a macro lens when photographed from in close.
This is not the case with normal lenses. The curvature of the lens elements means that the plane of focus for the lens is actually curved, too. Now for a lens that has a curve of, say ~1 cm, this is impossible to see when photographing objects at the usual distance of say 2 meters and beyond; because the depth of focus at that distance far exceeds the curvature of the lens so you would never notice the field of focus is itself curved.
When you get in really close, though, the depth of focus will quickly drop below 1 cm; and at this point, it is impossible to focus a normal lens so that both the center and the edges of the image are in focus at the same time. Using smaller apertures can help increase the overall depth of focus, but the problem of a curved field of focus is still there.
That's where true macro lenses, as 'flat field' lenses, come into their own. They are designed to provide a focal plane that is uniform from edge to edge, so what is in focus in the center will be in focus at the same distance around the edges of the image. If you are doing close-ups of small flat objects, such as stamps and banknotes and coins, a true flat field macro lens is essential.