When you hold you camera at hand, your hand shakes a little all the time, it is not something you can control. However, those little movements causes the camera to move also, and then, the narrow the field of vew, the larger displacement on the sensor pixel this will make. In the "film" age, the "rule" was that the minimal speed you shoudl use (hand held) with a certail lense is no smaller than the lens focal length. For example, a 200mm lense you should not (again - hand held) take pictures with shutter speed under 1/200sec. However, once the digital sensors went beyond the resolution of the film, it meant that you need to take pictures in a faster sutter to avode the "smear" cause by those little shakes. I could take pictures at 1/15 with the digital rebel (6.3MP), but that was impossible with the 7D. It depends on the sensor resolution and the angle of the lens, the wider the lens the less the effect. It is not a matter of "pixel size", it is a matter of the angular differnce they have in the picture, the smaller the angular differnces, the more those little shakes will be visible. You can test it yourself to see the effect on your own camera and what are your "body limits" in that field.