Originally posted by excanonfd This formula, I don't doubt the efficacy of it since I cannot offer anything to counter, but isn't it a bit simplistic? It doesn't take into account for the lens' physical size and weight. Here are two Tamron MF zoom sitting at the opposite ends of the Adaptall-2 lens line up. Tamron 159A, 70~210/4~5.6 and Tamron SP 30A, 80-200/2.8. The 159A weighs 353g and 30A weighs 1359g, nearly four times as much. According to the formula, the FL value would be 115mm for either of these lenses, this value might be sufficient for the former, but for the latter?
As I've said, I don't know enough about the subject to make an intelligent argument to counter the formula, but in my uninformed opinion this formula is incomplete.
Thanks,
Hi, excanonfd! Well, the SR system is a bit of a mystery to many people (and I myself learned something new - to me - just recently), and it's not something that's explained anywhere in any great detail.
However, here's broadly how it works:
There are sensors in the camera which measure the amount of shake (actually rotational accelerations in up to 3 axes - pitch, yaw, and roll in the case of the K5/7). These accelerations need to be examined by the camera in order for it to determine velocities.
When it has the correct velocity info, then it has the ability to make the camera image sensor move at just the right velocity so that the movement of the lens's projected image is exactly tracked. But, in order for it to do this, it needs to know one more thing: the focal length of the lens. That's why it prompts the user for this info when it doesn't recognise the lens.
Note that all of this has nothing to do with the mass of the lens; a heavier lens might damp out shake, or it may induce it, but it doesn't matter - the camera just measures it and takes the steps necessary to compensate. If it knows the lens's FL that is...