A macro-lens only focuses on a flat surface because the optical
designers worked very hard.
Think about the surface of most lenses, it's a section of a sphere. It
wants to bring things into focus that are equidistant from that
spherical surface.
Optical designers add elements and glass with special refractive indexes
to "trick" the light rays into focusing on a flat surface.
Macro lenses are flat-field for two reasons. First, because copy
photography is such a common thing to do, there is great demand for a
lens that will focus, relatively close-up on a flat surface so the lens
manufacturers get busy. Second, their optical formula is very
conservative (usually no faster than f/2.8) making it easier for the
designers to get the flat field. They also corrected the lens for
optimum performance close-up, rather than at infinity as they would for
general purpose lenses.
Macro lenses are not flat-field because of their nature, it's because of
the demand for their design.
Mr. Bill
Bob wrote:
What about a true macro lens? Isn't that the only lens that will focus
on a flat surface?
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