Recently found EF 17-40 F/4L can also go 1.02 with ET25 attached. My lens is only few centimeters from my monitor before fill an large icon full frame.
How do I calculate the magnification ratio of any focal length, focus distance and ET 25mm?
Magnification ratio added = ET length / focal lengh.
E.g. A 25mm ET on a 50mm lens will add on an extra ratio of 1:2 i.e. half. So, if the lens already has a magnification ratio of 1:5, then total magnification = 0.5 + 0.2 = 0.7.
