In 35-mm film photography, the generally accepted meaning of "macro" used to be "capable of 1:1 magnification in the image plane".

This, however, is not a good definition when we are talking of cameras with different sensor sizes. The "gurus" who proclaim, for example, that a Four Thirds lens is not really a "real" macro one because of nopt fitting this definition, only prove that sometimes being half-literate in some area is worse than being not literate at all; the latter at least stops people from voicing authoritative opinions.

Consider a hypothetical camera with a 0.3×0.4 mm sensor size (it is not important if such a camera is possible; I'm just using it as an extreme example), capable of delivering images printable and viewable in customary sizes (paper, screen). Nobody cares how big the sensor really is, as long as the resulting file is OK for printing and viewing. Nobody views the image directly on the sensor. If the camera can fill a frame 0.6×0.8 mm in size, a guru like this will still say it is not a "real" macro, because the magnification in the image plane is "only" 0.5×. With the image totally filled in with a mugshot of a flea?

On the other hand, a full-plate view camera (18×24 cm) will, at the unary image magnification in the image plane, fill the frame with your model's head, being not capable of getting any closer. Using the same definition we would have to say, that it has the same macro capability as a 35-mm camera covering the field 36 mm across.

Thus, our semi-literate guru would have to arrive to conclusion, that a view camera covering a view 24 cm across has the same macro capability as a 35-mm one covering 36 mm, twice as good as our hypothetical digital covering 8 mm. Come on, something is plain wrong here!

The only way out of this mess, is a definition which is independent on the sensor or film frame size.