Originally Posted by Zodduska
I don't really get the comments regarding high shutter speed with flash. As far as I understand it when using flash it is the flash which freezes the subject, makes a clear shot and it is indifferent to shutter speed.. as opposed to shooting constant ambient light where shutter speed matters. AFAIK the only effect shutter speed has when using flash as primary light is the amount of ambient light allowed to be captured. In most cases a higher shutter speed with flash will only render a darker background not a sharper image. (edit: this is based on my basic total newbie understanding of indoor flash photography)
For a newbie you have a pretty good understanding of the concept. What you are describing early on, is where the capacity to freeze the subject is determined NOT by shutter speed, but by the duration of the actual flash. In those cases although the shutter might be open for all of the 1/10th or 1/30th of a second that you tried, but the strobe is probably reaching the brightness needed to expose for f8 for only about 1/500th sec or even faster. The flash duration varies according to the actual strobe unit and the amount of power being delivered to it. At higher powers the duration is usually longer. Flash duration is usually in hundreths of a second, and can be much faster (your test shots look like your strobe is firing at a faster duration, which would make perfect sense if the strobe were on "auto" and you are pretty close to that fan - try the same experiment with the strobe on "manual" putting out full power every time and you will probably see more blur to the blades. You will need to adjust your aperture as the output will be much brighter). It is no surprise at all that the two pics you did at slower speeds with strobe as the main source froze the motion as they are probably primarily lit by the very brief duration of the strobe. In the case of your tests, you would get no sense of blur fron the ambient exposure if the exposures you used were several stops down from what the ambient light was reading.
The Auto FP setting is meant primarily for using strobe as a fill-light for outdoor use. It allows you to use the higher speeds to get an appropriate exposure for daylight while having an opening wide enough to allow a relatively low-powered strobe to fill in the shadows at moderate distances. You describe how it works perfectly - the strobe stays illuminated at a lower power for the entire length of the exposure.
It's great that you are doing the experiments you are to understand the concept. Nothing will allow you to grasp a notion faster than actually doing it.