If I have a macro setup consisting of more than a normal 1:1 lens (tubes, reversal, or anything else), 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?
After a little research online, I have found two corroborating pieces of info.
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.
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.
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.
It is quite simple, just follow these steps:
That will give you the true magnification ratio of your setup. If you want an estimate, just round the size of your sensor ;)
There is a silly catch if you are doing this to compare with manufacturer's specifications of lenses. The quoted magnification ratio (ex: 1:1 or 1:2) is based on the lens being placed on a camera with the largest possible imaging circle. So a full-frame lens on a full-frame body gives the quoted magnification. However, for a full-frame lens on a cropped-sensor body, you have to multiple the reproduction ratio by the crop-factor.