GH3 Lumix 12-35mm Rack Focusing Loses Focus

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Benjobe

Well-known member
I've had a Lumix X Vario 12-35mm f2.8 lens for a few weeks now and I've been experiencing an intermittent problem. Every once in a while, like every other day, I will zoom in to 35mm and focus on my subject, then I will zoom out to my desired focal length and I will find that my subject is out of focus. This will happen two or three times in a row before suddenly the phenomenon disappears and my subject will remain in focus.

Sometimes switching to from AFS to MF will solve the problem. One time it seemed to cease after disabling OIS. I've searched these forums for answers but couldn't find any. Anyone have any similar experiences, or have any idea what's happening with my lens?
 
You know what, I'm having exact same problem with my 35-100, which is always on MF. Normally on a video camera, common practice is to zoom in, focus, zoom out. I CAN"T do that, as I found out on a shoot a while back, as it will go soft if I change the shot. That is a giant pain as I have to rely on the x4 expanded focus at the set zoom point. Not great design for any lens.
 
Neither lens is advertised as being parfocal (focus point stays the same regardless of focal length), so I don't think it's fair to expect them to perform as such.
 
I thought that "parfocal" meant that the image would remain in focus during the zoom movement, regardless of the stationary extremes of each movement. By this I mean that the image would be in focus before the zoom movement, would be out of focus during the zoom movement, and then come back into focus at the end of the zoom movement. This is what I have experienced with other Lumix lenses, like the 75-200mm and 100-300mm.
 
Not sure if I really follow your description Benjobe. When a lens is parfocal you can zoom in, focus, zoom out and then lens will still be in focus. When a lens is not parfocal zooming will often cause it to lose focus. The 12-35 is not parfocal (very few none-cinema lenses are). This is part of the reason why focus peaking (which the GH3 doesn't have), focus assist, and loupes or EVF's are useful things.
 
woodcarver said:
Not sure if I really follow your description Benjobe. When a lens is parfocal you can zoom in, focus, zoom out and then lens will still be in focus. When a lens is not parfocal zooming will often cause it to lose focus. The 12-35 is not parfocal (very few none-cinema lenses are). This is part of the reason why focus peaking (which the GH3 doesn't have), focus assist, and loupes or EVF's are useful things.
Click to expand...

Carefull woodcarver, mention focus peaking and the GH3 in the same sentence and Malibu will bite your head off. Apparently anyone who needs focus peaking doesn't know what they're doing ;-)

To the OP, as has been mentioned, the 12-35 is not par focal. Parfocal doesn't just mean that the lens will loose focus slightly during zooming and then return back in focus. It means that once you zoom the lens, focus cannot be guaranteed and so the method of zooming in focusing and zooming back out is useless. The magnified focus box helps greatly but I agree that Peaking is a much needed feature on the GH3.
 
Ok, well then that's good to know. However, I find it strange that sometimes it keeps in focus while other times it doesn't.

In any case, thanks for everyone's help.
 
According to this video the 12-35 may loose focus _while_ zooming. But it seems to remember exactly where you focused on at 35mm. So it's absolutely possible to zoom it, focus and zoom out again to reframe your shot. I do this all the time.

- http://vimeo.com/50671577

(The test was done on an AF101)
 
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