Thanks useful information: 
True macro lenses will allow focus to reproduce the subject in a 1:1 ratio on the sensor plane.Originally Posted by cameronpatrol
That is, your 5mm subject will be photographed across 5mm of the sensor(or film if you prefer).
Some time back, it was agreed by some photographic body and the manufacturers that the macro designation could be used on lenses that magnify the subject by up to a ratio of 1:3.
That is, 1/3rd magnification.
My understanding of what the term macro actually means is a reproduction ratio of between 1:1 and about 6:1(that's 6x magnification).
Beyond 6x magnification the term become microscopy, which is not the same term as Nikon's 'Micro' designation of it's lenses.
Not all 'proper' macro lenses will allow focus down to 1:1, and even these previously referred too prime lenses.
Ziess, Voightlander (amongst others) have high quality macro designated prime lenses that can only do 1:2 reproduction natively and require extension tubes to go to 1:1.
I can't think of any zoom type macro lens that can do 1:1 reproduction without the use of any extension.
That I can think of, only one lens currently and easily available to the public is a true macro lens, and that is the Canon MPE which I think does 1:1 and higher magnification.(I can't remember all the specs of this lens, others have more knowledge of it).
Reproduction ratio is stated in a few ways.
X:Y is one way(eg. 1:3),
and N.NNx is another(eg. 0.33x) .. where the x means times, not X, and is the maximum magnification ability of the lens.
As an example of this, if you look at the specs of the Sigma 17-70 macro lens, it's maximum reproduction ability is 1:2.9, which is basically 1:3, which turns out to be 0.344x magnification
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