I am trying to buy a macro lens, and I would like to know how the 1:1 reproduction ratio is measured or where it is determined from, etc.
So where is the ratio measured from or what determines a 1:1 reproduction ratio?
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The 1:1 ratio means you can focus close enough that you can fill the frame with an object the same size as the sensor. I.e. for an APS-C DSLR this means you can focus on an object only 22mm wide.
The 1:1 reproduction ratio is independent of focal length, you can have a 50mm macro and a 200mm macro both filling the frame with the same 22mm wide object, albeit at different distances.