I agree to all the above arguments. I may have chosen my words sloppily.


As far as I understand it, it should be like this (I hope I'm not totally wrong, which I may be, I must admit):


The extension tube does not change the focal length of the lens, because this is fixed by the lens elements in the lens and an extension ring does not change that. But it does move the principal plane of the lens farther away from the focal plane (the sensor). This should have an effect on the angle of view: It should get more narrow, and thus the effect should be *like* en extension of focal length of the lens. Also, by this the image circle should increase (I believe) probably illuminating a full frame sensor.


I know the calculations are only true for focussing to infinity (i.e. parallel rays), but I calculate it like this:


focal length = distance of the principal plane of the lens from the sensor (for parallel rays), say zoomed to 10mm the principal plane would be 10mm away from the sensor.


Add a 12mm extension tube and this distance will be 22mm, so the "focal length" should be around 22mm - for focus to infinity, which, however, is not possible any more (this might be the catch of my over-the-thumb calculation), because the focus ring will be at its stop.


With my statement "there is no point in that" I ment that you will not have a setup that permits you to do landscape or indoor photography. I sort of assumed this was the principal purpose of an extreme wide angle lens.


Should I be totally wrong with my line-of-arguing, I'll take everything back and state the exact opposite!