C100: Sigma 18-35mm f1.8

Regarding being parfocal, my guess is that it may have more to do with variation among lenses. Based on my experience, if the manufacturer isn't precisely calibrating the lens to be parfocal during assembly, then it is all a matter of luck on whether your copy will turn out to be parfocal. It's an extremely small difference in alignment that can lead to shifting focus during zooming, and I've read that the manufacturers no longer take the time to align the lens properly even if the lens could be parfocal since the number of people in the photo world that care is extremely small.
 
IIRC, even parfocal lenses aren't parfocal unless they're back focused. What lens do you have that has a back focus adjustment? I haven't seen that since the ENG lenses on our old Ikegamis.
 
Sony XDCAM HD camera with Ikegami HD Broadcast lens. Still very much a part of my broadcast business, sports, news, broadcast tv production. For all the Commercials, and creative I have been shooting with a Canon C300 and C100.
 
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