Well if you want to forget about output size then what you mean is that the image is as sharp at 100% as it would have been at a faster shutter speed.
I'm pretty sure we (i.e. I) was the one that pioneered the 1/(2xShutter) rule of thumb here and I maintain it as a good starting point, but the exact rule is dependent on 1) your camera body, 2) your camera body settings, 3) how sturdy your hands/stance are and 4) the pixel pitch of your digital back.
We just ran a test for Stu Baserman with the P65+. He was concerned about hand holding with the extra resolution. As expected camera shake was introduced about 15% (hard to be especially accurate because of shot to shot variation) sooner compared to the P45+ as you dropped in shutter speed.
What I'd like to point out though is that this is only to get to 100% sharpness for a given system. Having a full pixel of camera shake on the P65+ may look soft when viewed at 100% but it will still produce an image with an effective 15 megapixels of information.
Below are two 100% crops from those shots. The right image is at 1/80th (f/13**) with the Phase One 80mm lens. It looks pretty sharp in isolation (especially if you're used to viewing dSLR files), but then you compare it to the 1/160th (f9.5) and you realize the 1/80th has a haze of camera shake. Still, the 1/80th could be printed, and could even be printed large, just not as large as the 1/160th.
These are from a screen grab at 100% in 4.5 Pro with default sharpening and no clarity adjustments.
The reason you need higher shutter speed than with a film, from which the 1/shutter rule came, is first and foremost because the effective pixel density is higher*. Camera shake is a function of the number of pixels the subject moves across during the exposure. It's not that camera shake produces a worse image with a P65+ compared to say a P25+ (60.5mp and 22mp respectively). In fact, if you take an 80mm lens and shoot each at say 1/160th hand-held and print them the same size you will find they look the exact same! However, if you can get to a faster shutter speed the P65+ will continue to gain additional sharpness while the P25+ will stay the same (as it is already maxed out on detail).
In fact the same goes for film. If you shoot a digital back and an equivalent piece of film with 1/shutter as your rule of film then you could make an equally good final print. But that would be because the film is maxed out on sharpness while the digital back has been dumbed down to the sharpness level of the film.
Also I would be remiss if I didn't mention that the H3DII has one less stop of usable ISO range compared to the equivalent Phase One back for the H body (the P45+ can go to a very usable ISO 800). A full stop of ISO is more important than any other consideration when talking about camera shake.
Doug Peterson, Head of Technical Services
Capture Integration, Phase One Dealer
Personal Portfolio
*depending on your body MF may have a more severe mirror slap compared to older film bodies or dSLRs, but this is often counter acted by the additional mass of the body which dampens (or translates to lower frequencies) the shake from your body
**diffraction is a slight concern at f13, but I ran other test shots to confirm that the softness in this image far exceeds the minor loss of sharpness due to diffraction
