Hi Celsius,
I assume youre referring to my
Thread .
The magnification you gain from Extension tube is dependent on both focal length and extension tube length. For subject below 20cm size, I normally stays at 70mm-100mm focal length, unless I want more working distance then Ill use 150mm-200mm of course magnification will be lesser.
Like what xdivider said, crop factor of 1.6X for DLSR plays an important role to increase the magnification as well. But, I always based my calculation using the standard formula we all know. (ExTube length / focal length)
For my above thread, it was shot at around 100mm, 150mm and 75mm repectively (with 10-20% cropping)
One of the best way to increase magnification is to use an Extender between the cam and the extension tube. In this matter, the extender will multiply your extension tube strength leaving the minimum focusing distance the same.
This was shot using 70-200mm + 25mm ext tube + 1.4X TC at 70mm focal length. (cropped about 5% only) Your 25mm now becomes 35mm giving you around 1/2X at 70mm focal length.