Sigma 120-400 OS users

Few questions :

• Have you noticed maybe bluring with speeds over 1/500 because of OS ? (There are stories supoorted even by Thom Hogan about VR causing such issues @ speeds faster than 1/500 http://www.bythom.com/nikon-vr.htm )
   
• Which OS mode is for panning ? 1 or 2 ? (is mabe "1" equivalent to VR "Normal" and "2" to VR"Active"?)
   
• Is 400 the actual measured length @ 400mm ? (or maybe smth like 380mm ?)
   
• Which F is the optimum for this lens across the focal range? F8 ? 

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