Class A, I think it's all clear. So my response is meant for you and I'll try to be brief now.
Originally posted by Class A I know. It was meant to be a hypothetical assumption. I should have written "were infinity".
You cannot make this assumption, not even hypothetically. As soon as the length of extension tubes is larger than zero the subject distance
must become finite. Therefore, you miss to understand that the formula already takes focussing into account.
Quote: Your equation is useful to illustrate the change in focal length but doesn't take focusing into account.
My equation describes the effect of focusing. You must miss something.
Quote: Why is your original answer "no"?
It is no to the original question "if [...] 50mm [...] should still have 50 entered for shake reduction?"
Quote: The combined system lens+tube just acts as if it were a lens with a different focal length. No?
No. The extension tube does nothing. There is no glass in it! It only allows to focus closer to the subject. Maybe, you better replace the extension tube by a bellow in your imagination.
Quote: primarily it is designed for compensating translational shake. Can you please point us to your source that says otherwise?
As soon as you have an in-depth understanding of how the SR system works you'll see for yourself. The Pentax can only measure angular velocities around two axes and the sensor can be translated in two directions and rotated around one axis. However, because the camera cannot measure translational shake, the sensor translations are computed to compensate the rotational shake only (which the focal length is required for). Translational shake can be ignored for large enough subject distances. You find all the nasty details somewhere else, including some posts of myself.
Quote: Perhaps you are referring to the fact that the sensor is rotated around the X- and the Y- axes to compensate shake?
The sensor isn't rotated.
I don't want to sound arrogant, but I will stop discussing this now. I wanted to give the OP some help about how to achieve less shake when using extension tubes.