Originally posted by yusuf Where did you read about 2x and that formula? This topic has been discussed to death but why don't you share you experience with pictures, set your zoom on 70mm, with & without SR at 120mm.
I didn't read about it, I derived it. Done to death? Only because some people seem to suspend analytical thinking, in favour of hearsay. I suspect you did the following (using a 70-210 as an example): you tried a few shots at 210mm actual, with a 70mm set value. Well, you'll get a 30% improvement in shake-induced blur, but how would you notice (unless you'd taken 100 shots)? Then you'll have tried a set value of 210mm with a set value of 70mm. So you get 2x shake magnification (which is exactly what I'm warning against), and you conclude that it's all useless.
It's actually quite easy to explain. Take a lens of focal length 100mm. Shake causes image to move by 1mm across the sensor plane:
SR off: sensor sees 1mm movement: 1mm of blur.
SR on, FL value set to 50mm: sensor moves 0.5mm: a net 0.5mm of blur
SR on: FL value set to 100mm: sensor moves 1.0mm: a net zero blur
SR on, FL value set to 150mm: sensor moves 1.5mm: a net 0.5mm of blur
SR on, FL value set to 200mm: sensor moves 2.0mm: a net 1.0mm of blur (same as SR-off)
SR on, FL value set to 300mm: sensor moves 3.0mm: a net 2.0mm of blur (2x blur of SR-off)
I have posted several times on this very issue, but I'm not sure I can be bothered to exhume them - and you're only going to embarrass yourself.
Try the formula. You might like it. But don't expect miracles: SR is extremely effective when FL is matched. You'll get only a relatively small improvement at the extremes of the zoom lens - but still worthwhile, and a lot better than turning SR off. But of course, you've given up an Pentax cameras anyway.
A word to the OP: why are you considering a vintage 70-210? They're mount-threateningly heavy, and expensive too (because the Vivitars are cult items). What's wrong with a Pentax M75-150 - much smaller/lighter, easily as sharp, much cheaper and you'll suffer less from a focal length mismatch.