I've been thoroughly enjoying my new K-5 for about 6 weeks now. After doing some indoor shooting handheld, and running into the usual compromise between shutter speed to control camera movement and aperture to improve sharpness, I decided to test just how far I could rely on the camera's in-body shake reduction circuitry. So, I began shooting a nearby fire extinguisher at higher and higher F numbers, and slower and slower shutter speeds, until I began seeing motion-related blur. It remained razor sharp right down to about 1/20 sec, at which point I saw the tiniest increase in motion effects. But as I continued to longer and longer shutter times, the effect hardly increased at all. In fact I began wondering if I can really seen any effect. Then, suddenly, at 1s exposure the anti-shake finally failed and motion blur was very obvious and extreme. Amazing! This is better than even the Panasonic cameras I've used, and I had been under the impression that Panasonic was absolutely the best as far as image stabilization.
So, I backed off a bit and checked the results at the slowest shutter speed which seemed to be tolerated - 0.5s, with and without the anti-shake function enabled. The results speak for themselves, below. To say I am amazed is almost an understatement. What a difference this makes. I love this old Zeiss lens, and am thrilled I can get so many advantages with it on the Pentax body. I can just hear the marketing types at Canon and Nikon talking about anti-shake: "Shall we put the circuitry in the camera body, so that people can use our best lenses from the past 50 years? No. That would cut into our sales of new lenses. Make them buy new lenses. If they bought our camera, they can surely afford the lenses." And what does Pentax say? "Putting the anti-shake in the camera body is the proper engineering decision, even if we have to charge a little more for the camera. Pentax users will love our new camera if they can use their best lenses from the 60's, 70's, and 80's." You got that right, Pentax.
Detail: Pentax K-5, 0.5s, f/11, ISO 100, Zeiss T* 50mm f/1.4 lens, indoors under fluorescent light.
Below: the original frame, the one with shake-reduction, scaled down to 800x530px. Pentax K-5, 0.5s, f/11, ISO 100, Zeiss T* 50mm f/1.4 lens, indoors under fluorescent light.