Wednesday, September 1, 2010

Macro Magnification - how to calculate?

This post is all about how to calculate/measure  macro magnification. It's really very simple but since I get asked this a lot, I might as well prepare a post on this for everyone's benefit. A lot of macro photographers add Raynox DCR250 and/or Extension Tubes and/or Teleconverter to their macro lenses (or non macro lenses) and wonder what magnification they are shooting at. Well, I say, shoot an mm ruler.

But first, let's get a couple of definitions out of the way.

1:1 (or Life Size)

If your subject is 22mm long, and it fills up the width of your crop sensor's frame, then it's 1:1 or life size. For a Canon crop sensor DSLR, such as the Canon 40D, the sensor size is 22.2 x 14.8 mm. A great site to check for specifications like this is That is where i got my sensor size:

 You can even find it by just googling "40D sensor size" :D.

 So by looking at the ruler shot on the left, you see approximately 22mm there, therefore it's 1:1 (life size).

Magnification = sensor width / # of mm captured
                       = 22.2/22
                       = 1X (approx)

Magnification = sensor width / # of mm captured
                       = 22.2/11
                       = 2X (approx)

Picture4Magnification = sensor width / # of mm captured
                       = 22.2/7
                       =  3X (approx)

Yes, it's that simple :)

Another illustration just to show you the different magnifications when you shoot a real life subject at different magnification and also with a FF (Full Frame) camera. Let us assume the soldier fly image was shot with a 40D (crop sensor) at 1:1. The yellow, red and blue frames are what they would look at at 2X, 3X (crop sensor) and 1X (FF) respectively.

print magnification vs optical magnification soldier fly

Based on the input from a few forum members, you can get up to 2:1 by adding a full set of 68mm of Kenko Extension Tubes to a 90mm, 100mm, and 150mm macro lens.

Theoretically, the calculation goes like this:

Magnification gained = # of mm of tubes used / focal length

so if you used all 68mm of tubes on say a 100mm, you should gain an additional 68/100 = 0.68X

Add the native 1X from the macro lens and you get 1.68X. However, the actual measurement tells us you can get up to 2:1 magnification.

It is even more surprising to hear that you can get up to 2:1 too on the 150mm.

Okay, I know you magnification junkies out there won't be happy with 2:1, you want more. You can still add a 1.4X Teleconverter to your setup and get up to about 2.5X. Actual macro magnification may vary so again, go shoot an mm ruler:D

So the best way to calculate/measure macro magnification is to shoot an mm ruler. Period :)

Oh i assume you already know how to achieve higher magnification. There are many ways, but I personally prefer the use of extension tubes, as discussed here, and a 1.4x teleconverter too, if you want to get even higher magnification. Diopter lens like the Raynox DCR250 is okay too but i prefer extension tubes better.


  1. Hi Kurt, I have always thought that the longer the focal length, the higher the magnification you can get with extension tube, but its seems that actually it works the other way around. Does this mean that if you use the full set of 68mm extension tube on a 35mm f1.8, then the magnification would be 1.94X!!!

  2. Hi Sani, no, ET gives you higher magnification gain on shorter focal length. 68mm of tubes on 35mm should give you close to 2:1 but again, it's better to just shoot a ruler to confirm :). I suspect the working distance will be very small too.

  3. Now I can understand a little bit of magnification..thanks sir..



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