How do I measure the magnification ratio of an unknown macro setup?
Asked 6/20/2011
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2 answers
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I have a macro setup that may include extension tubes, a reversed lens, or other accessories, so I do not know the actual reproduction ratio. What is a practical way to measure the true magnification ratio of the setup, such as 1:2, 1:1, 2:1, or higher?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
6
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.
Originally by user3205. Source · Licensed CC BY-SA 4.0
user3205
15y ago
0
Generated from our catalog & community — verify before relying on it.
A simple way is to photograph a ruler and compare how much of it fits across the frame.
- Set the setup to its closest focus or the working position you want to measure.
- Place a ruler with millimeter markings flat and square to the camera.
- Move the camera until the ruler is in focus without changing focus.
- Take a photo.
- Measure how many millimeters of the ruler are visible across the image width.
Magnification is:
sensor width ÷ subject width captured
Examples:
- If a full-frame sensor (36mm wide) captures 36mm of ruler across the frame, that is 1:1.
- If it captures 18mm, that is 2:1.
- If it captures 72mm, that is 1:2.
- For Nikon DX, use 23.6mm as the sensor width.
So if 11.8mm of ruler fits across a Nikon DX frame, magnification is 23.6 ÷ 11.8 = 2, or 2:1.
This gives the true magnification of your camera-plus-lens setup. Be aware that manufacturer magnification specs are typically referenced to the lens’s intended image format, so crop-sensor framing can make the field of view seem tighter even though the optical magnification is measured at the sensor plane.
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