Calculating the actual size of something in the land of macro photography

Red mite photographed at 1:1 reproduction ratio (RR) on an
APS-C Nikon camera with a 20 mm extension tube.

There are a number of ways to calculate image size and magnification, but all of them rely on using math.  One of the problems is on viewing the final image because its size will ultimately determine the degree of magnification.  This, however, plays no part in determining the subject's actual physical size.  That process is a much simpler one.

In the above photo, we know that the horizontal distance across the image is 24 mm.  We can set up a simple cross-multiplication scheme to determine its actual size.  We need to know three values:  distance across the sensor (24mm), distance across the image (measure in = 508 mm) and the image distance of the subject (36 mm).  

Calculating the size of the mite using cross multiplication.

The mite's body is 1.7 mm long.  We could also determine the overall distance, distance with legs spread out, and so on.  Each would require the measurement of the parts of the mite on the image.

How did I get the measurements?  The original image was a 24 mp frame that the computer presents at 72 dpi (dots per inch).  Photoshop allows me to measure the distance directly using the properties tab combined with the selection or line tool.  I could have used a ruler on the screen, or printed the image and measured it that way, but this was far better.

Determining magnification can be done using image size/actual size.  If you want to determine magnifaction as a reproduction ratio, you can use the other information presented.  In this case it would be reproduction ratio of the lens (1:1) times the crop factor as a ratio (1.5:1) times the magnification from the extension tube (1.2:1) which gives a reproduction ratio of 1.8:1 or a magnification of 1.8x.

Com'mon - everybody say, "Math is fun!"

Thanks for reading.

Eric Svendsen     www.ericspix.com

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