How can I estimate the reproduction ratio of an unknown macro setup?

Question

If I have a macro setup consisting of more than a normal 1:1 lens...IE tubes, reversal, etc - how can I measure the reproduction ratio (2:1, 1:1, 1:2, 3:1 etc)? What process can I follow to accurately gauge it?

Answer

After a little research online, I have found two corroborating pieces of info.

Firstly:

The shorter the focal length of the lens used, the more magnification results. A 50mm reversed will give about life-size reproduction, while a 20mm gives 3X or 4X.

http://www.cameraontheroad.com/2000/07/closeup-photography-ways-to-get-close/

Secondly:

the most accurate way is to reverse mount the lens in question and take a photo of a ruler with millimeter markings. Then, for Nikon DX, divide 23.6 by the number of millimeters that fit, horizontally, within the frame.

http://forums.dpreview.com/forums/readflat.asp?forum=1034&message=35927198&changemode=1

It so happens that I have a reversing ring, a 50mm lens and a Nikon DX format camera, so I grabbed a ruler and lined up a shot. Sure enough, I measured approximately 23mm on the ruler, confirming a 1:1 ratio. Switching to my 18-55mm @ approximately 20mm, I measured 6mm on the ruler: 23.6/6 = 3.933, so pretty much 4:1.

So it would appear that the method suggested in the DPReview forum answer is correct.

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