Originally posted by Lowell Goudge pixel size, pixel pitch and pixel density have nothing at all to do with shake reduction and shutter speed.
lets go back to the basis of the rule of thumb, which was for 35mm film cameras shutter speed should be 1/focal length, to get a full frame image when enlarged to 8 x 10, where there is no "acceptable blur"
acceptable blur was defined as a point of light remaining smaller than 0.01 inch in diameter, or what was considered when viewed from arms length as indistinguishable from a true point. (note this is the same criteria used for acceptably sharp when discussing depth of field)
when you consider an APS-C camera, you need to add the crop factor, because the enlargement of the basic image is 50% higher, to get back to an 8 x 10 inch print.
if you are viewing at the pixel level, you are enlarging at much higher magnifications and it is not how you should measure shake reduction efficiency. in this case, you need to add, in the shutter speed calculation (basic rule of thumb) the linear magnification ratio compared to the base 8 x 10 reference used for acceptable sharpness.
you also need to consider that for the K1, being a full frame camera, for example, that the shake reduction may appear to be more effective because the final magnification of the sensor image to the print is less than for a cropped sensor
This may be how this is counted, but this is not how people will count it. People will go look at the 100% crop and see if this is sharp or not. Even between 2 bodies with same number of pixels like K5 and K5-IIs the removal of the low pass filter will make a difference. As well as the number of pixels, because this increase magnification.
The sensor format should not be a factor at all to me, because when switching format, it make sense to speak of equivalence: similar field of view. This mean same requirements for SR by your own definition. So no change at all.
But for user perception when looking at a 100 crop, K1 has more pixels, so people will notice blur sooner whatever the official way of measuring SR.
Anyway the efficiancy depend of what type of shake you have, the focal length and actual shutter speed. The claim always seems to be a bit optimistic. 4.5 stop mean I should be able to have a sharp image on APSC with 15mm handed at 1s shutter time, a 50mm at 1/3s shutter time or a 300mm at 1/20... This look to be a bit optimistic for me. In practice SR seems to be more about 1.5-2.5 stop to me, it being more effective with longer focal length. That's already quite nice.