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Extension tube magnification calculations

Just some comments on my understanding of the magnification achieved when using extension rings on macro and non- macro lenses.
With normal non- macro lenses, magnification seems straight forward. You can use the formula tube length (mm)/ lens focal length mm and get an answer that seems to work.
So for example I recently used an emergency macro lens made up from a standard 50mm lens and 75mm of rings, this gave me 75/50= 1.5:1 magnification which agreed with what I saw in the viewfinder. The sensor on my camera is about 22mm wide and at 1:1 I can see about 22mm of a ruler throught the viewfinder. In the above case with the 50mm lens and 75mm of rings, I could see about 15mm of a ruler in the viewfinder so the magnification was actually 22/15= 1.47:1.
What about macro lenses being used at 1:1?
This gets more complicated. The formula changes for magnification to include the notional length of extension rings the macro lens already has built in to achieve 1:1- this is the same as the focal length.
So the formula becomes.
(macro lens focal length(mm)+ tube length (mm))/macro lens focal length (mm).
Now the interesting thing is that in practice this formula does not work very well.

My 105mm ex lens with 65mm of tubes, should have a magnification factor of (105+65)/105= 1.62:1.
It actually gives 1.97:1
Someone else supplied the figures for their canon 100mm macro lens- this with 68mm of tubes should give (100+68)/100 = 1.68:1
but actually gives 2.14:1.

I think part of the reason for the difference is that normal lenses do not change focal length much going from infinity focus (where the focal length is measured) to minimum focus which is where they are being used for macro. This is not true for macro lenses where the focal length changes considerably, so my 105mm macro lens is actually about 78mm at 1:1 and a 100mm canon macro lens is actually about 75mm at 1:1. The focal length of a macro lens at 1:1 approximates to the minimum focal distance/4. However even using this revised focal length in the magnification calculations still gives numbers that are lower than actually observed.

Enough of the rambling- I would be grateful for any comments or clarifications on this :)
Brian V.
What about the original extension of the macro lens? There's plenty of it at 1:1 ratio even without an extension tube added: I think it's twice the actual focal length at 1:1.

Let's take the Canon lens: ((2x75)+68)/100 = 2,18:1. Pretty close to the observation, I'd say.

Christian
so... if you add 1.4tc on the canon 100mm lens with the 68mm extension tube... how does that work out?
As long as you put the TC on first at min focus you'd get about 3:1 (1.4 * 2.14)

brian V.
Thank you Brian V, I had a picture of a dime using 40D + 1.4x TC + 68mm extension tubes using a 100mm macro. I guess-timated at about the same. heh but this topic helped me out. Thank you.'

What about with non-macro lenses like the Canon 135 f2 L with 68mm of extension tubes?
Jalexsmith - that would give 68/135 magnification at min focus ie about 0.5:1.
So what's going on if I use a 1:2 macro lens?

Ex: tube 50mm with a 100mm macro 1:2 lens, what magnification it will produce?

Thanks
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