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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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up vote 11 down vote accepted

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.

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Are lenses sold as 1:1 macro based on APS-C, or are they generally (or sometimes or always) specified in full-frame terms? – mattdm Oct 15 '12 at 17:37
@mattdm it doesn't matter what the sensor size is, a 1:1 lens will always let you fill the frame with objects the same size as the sensor. If you have a full frame camera and are filling the frame with a 36mm object, and then without moving the lens or refocussing, mount an APS-C camera, you'll only be able to see the middle two thirds of the object, but if you swap the object for something 22mm wide, then you'll be filling the frame again. – Matt Grum Oct 15 '12 at 18:02
Oh, right. That makes sense. Thanks. :) – mattdm Oct 15 '12 at 18:12

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