As dosdan was so kind to refer to in his post #3 (
https://www.pentaxforums.com/forums/1724938-post3.html ), SR with 300mm+ lenses on Pentax is effective.
As is confirmed by user experience here too.
Maybe, let me make a number of additional remarks:
- Pentax SR efficiency with long FL is about 2 stops, much in line with most long lenses optical stabilization efficiency. So, replacing SR by a Sigma OIS will most likely
not improve things.
- Pentax SR with short FL and long exposures such as 1/15s can be better, like 3+ stops. And at around 1/80s, the K-7 has the problem that SR cannot compensate for shutter-induced shake and
seems to be less efficient. At some point in the past, this confused discussion of the topic.
- Contrary to common belief, the sensor shift technology is
not limited by FL: This is easily seen: If you want to improve by 4 stops, you need to shift 2^4 x the circle of confusion which is only .16mm and independent of the FL. The higher FL is compensated by a shorter exposure time and both effects cancel out. However, longer FL requires higher acceleration (read stronger motor) and any particular implementation has its limit. Pentax' is at 800mm.
- SR works by electronic coriolis force sensors which measure angular velocities, not acceleration. This is why panning can be a problem.
- The Pentax SR mechanism needs a time to initialize (called "lock" earlier up in the thread), indicated by a hand-symbol in the viewfinder. During this time, it measures angular velocities and brings the sensor into a starting position. However, it only starts shifting as soon as you press the trigger. The initialization happens during half press. The K-5 then seems to stay initialized as long as the viewfinder remains illuminated.
- It is possible to fully press the trigger before SR is initialized. The result normally then is a blurry image. I've already lost a panorama because I turned and shot to quickly in succession with SR on. So, if you need to shoot extremely fast, better switch SR off. That's particularly true with super wide angle which can be shot at hyperfocal distance.