<p>This question came up today, and I researched the topic. There seem few scientific answers to the question, even though it's essentially a geometry problem with a number of variables.<br>
One interesting reference I found was the Rule of 600, which refers to what shutter speeds are needed to avoid star trails, which can be traced to the earth's rotational speed relative to the stars.<br>
There is also a generally accepted variation of the rule of thumb that cameras with crop sensors need to multiply the reciprocal focal length by the crop factor to avoid camera shake.<br>
Which brings me to the actual factors playing a role:<br>
Camera shake happens when natural hand movement makes an object that is static in the scene (i.e. has no movement of it's own) move multiple pixels on the sensor in the time the shutter is open. Obviously shorter exposure times mean the same object has to move faster. However, we can assume that movement inducing shutter speed is relatively stable, though differs from person to person (as mentioned), can be influenced by camera ergonomics, camera weight, and environmental factors.<br>
Comparing camera factors, one common data point is field of view, that allows us to compare various lenses and focal points. As it turns out, using focal length, and the existing rule of thumb, it appears for 35mm cameras and lenses, the rotational speed of the camera has to be kept below 2000 deg/s.<br>
Example: the 50mm focal length on a 35mm lens has a horizontal angle of view of 40 degrees. The rule of thumb would indicate a safe shutter speed of 0.02s. That results in 2000/s. If one checks other lenses and their respective rule of thumb exposure and angle of view, the result is pretty steady.<br>
Comparing this with a PhaseOne P65+ back and 645 set of lenses, the comparable 'normal' focal length is 80mm, which has an angle of view of 47 degrees. Assuming the same rotational speed the safe shutter speed would actually be 1/40s, not 1/80s as the rule of thumb would tell us. Now, this 80mm lens isn't the exact equivalent in terms of angles.<br>
So it stands to reason that the 1/50s safe shutter speed translates to whatever the normal focal length is in the respective system.<br>
Having said this, this will yield an equivalent result if the image were to be viewed at the same final size, disregarding that the MF image is made up of many more pixels.<br>
In fact motion blur will be result of perception. And it would be better to say, that any camera shake induced motion blur should not exceed x pixels of movement, where x is likely to be 1 or 2 for perfectionists, and maybe 5 for most people with discerning taste.<br>
So the 1/50s second on MF would still result in a higher pixel count movement (the 5DM3 has a vertical resolution of 3,480 pixels, compared to the P65+ with 6,732 pixels).<br>
So if one were to peek at the 100% magnification of the image file, the safe shutter speed for a P65+ would likely be more 1/100s for a normal focal length.<br>
PS: The mention to just use a tripod is valid, however it assumes photographs that lend itself to tripod photography, such as landscape or product. In other commercial genres like fashion a tripod is not practical.</p>
