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FORUMS Photography Talk by Genre Macro Talk 
Thread started 14 Apr 2009 (Tuesday) 13:39
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how to figure out ratio with ext. tubes?

 
badams
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Apr 14, 2009 13:39 |  #1

What is the ratio of a nifty fifty and ext. tubes (68mm)? Is there a formula so that I can figure out what my other lenses are?


Everyday use: 7D2, 1.4x v3 Canon TC (sometimes the 2x v2 Canon TC), Canon 500mm f4 L IS USM; 6D, 24-105L
http://www.southernokp​hotography.com (external link)

  
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bokchoi
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Apr 14, 2009 16:42 |  #2

The general formula for finding the magnification ratio is:

Maximum Magnification = Total lens extension / Focal Length

A bit more usefully, you can find the magnification ratio with a lightly modified version of the formula:

Maximum Magnification = (External Lens Extension / Focal Length) + Inherent Magnification

So, using Google to find the maximum magnification of the 50mm f/1.8 to be 0.15x, we can use the second formula to calculate the maximum magnification of the Nifty Fifty focused to its closest point and 68mm of extension tubes:

Maximum Magnification = (External Lens Extension / Focal Length) + Inherent Magnification

Maximum Magnification = (68mm / 50mm) + 0.15x

Maximum Magnification = 1.36x + 0.15x

Maximum Magnification = 1.51x
Therefore, your maximum magnification with the Nifty Fifty and a the full 68mm of extension tubes is 1.51:1, or 1.51x.


Another way to find your total magnification is to photograph a ruler; the magnification can then be found by dividing the Camera's sensor's size by the length that you were able to capture in the frame.

So, knowing that the 40D you are using has a sensor size of 22.2mm horizontal, you take a picture of a ruler, and see that you can fit about 14.7mm horizontally across the frame. You can then find your magnification to be:


Magnification = 22.2mm / 14.7mm

Magnification = 1.51x

Again, we get 1.51:1 or 1.51x. This is the same answer that we had using the previous calculations, which means that the lens must have been focused to its closest point.


Neither of these methods is a perfect way to calculate magnification due to all kinds of manufacturing variations and other factors, but in my experience, the results have always been accurate and reproducible enough for my purposes.

Hope this helps! :)



  
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badams
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Apr 14, 2009 23:49 |  #3

Thanks. Doesn't have to be perfect as close is good enough for me.


Everyday use: 7D2, 1.4x v3 Canon TC (sometimes the 2x v2 Canon TC), Canon 500mm f4 L IS USM; 6D, 24-105L
http://www.southernokp​hotography.com (external link)

  
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averylowiq
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Apr 15, 2009 02:05 |  #4

Thanks for taking the time to write bokchoi. Your post is very useful.


http://www.flickr.com/​photos/15122283@N05 (external link)

  
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how to figure out ratio with ext. tubes?
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