Alley in Pisa, Italy

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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?

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2 Answers 2

up vote 6 down vote accepted

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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How close to your subject do you find yourself with it reversed at 20mm? –  rfusca Jun 20 '11 at 16:48
    
Very. Next time I've got my kit out I'll measure it. –  ElendilTheTall Jun 20 '11 at 20:22
    
It almost goes w/o saying that the ruler has to be perpendicular to the lens axis! –  ysap Sep 9 '11 at 20:48

It is quite simple, just follow these steps:

  • Focus as close as possible.
  • Place your camera so that it sees straight on to a rule.
  • Move it closer until the ruler is in focus without adjusting the camera focus.
  • Take a photo.
  • Divide the width of the ruler as seen in the photo, by the size of your sensor.

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.

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3  
I don't think the crop factor matters. You're just capturing less of the same image circle — you'll get a wider image on full-frame, but the magnification should be the same. That is, 36÷36 and 24÷24 both equal 1. –  mattdm Jun 20 '11 at 13:34
    
Yes, the magnification is the lens magnifies the same but you capture a small areas. So the only case where it is relevant is the (36/24 case). That is why Olympus specs of the Zuiko 50mm Macro say 0.52x (equivalent to 1x life-size on a 35mm camera). –  Itai Jun 20 '11 at 13:44
1  
I don't understand your comment, however I agree with mattdm, with a smaller sensor you capture less of the ruler, but you divide that measurement by a smaller number so the magnification factor is the same regardless of sensor size. –  Matt Grum Jun 20 '11 at 16:00
    
Yes, I agree with both of you. We are referring thought to what I put under silly catch: That manufacturer's magnification spec are sometimes adjusted for sensor-size even if the lens would cover the imaging circle! –  Itai Jun 20 '11 at 16:06
    
The 0.5 in Zuiko 50mm specs is the correct actual reproduction ratio, it does not have to be multiplied by crop factor. Size of sensor cannot affect magnification of a lens, which is a purely optical phenomenon. Magnification ratio at a fixed flange distance stays the same whichever size or shape the imaging area is. –  Imre Jan 3 '12 at 21:21

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