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 Dpreview.com. That is where i got my sensor size: http://www.dpreview.com/reviews/canoneos40d/page2.asp

 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)

Magnification = 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.

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.