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Follow Focus and drift-- advice?
  • Last night I shot a theater performance with my GH2. But the event was lit for theater, not video, so it was very dim. So I borrowed a fast Lumix 20mm prime lens. This also meant a shallow DOF, so this was a chance to try using the Fotga follow-focus rig.

    I ran into a problem. I marked the furthest and nearest focus points on the Fotga's white wheel. That way, I could focus based on the distance on the wheel marks. However, I tried racking the focus back and forth a bit, to get a feel for the thing... and the settings drifted. I'd rack the focus to the mark for the back of the stage... and it wasn't in focus. Same thing for the nearest point. I'd recalibrate, but the points would drift again.

    Now, there's a bit of play with the Fotga follow-focus-- and I was using a small 12mm lever that added a little more play-- but I'm wondering if there's some issue with the lens or camera settings that may have contributed.

    (One thing I did learn: as you rack focus during shooting, the GH2 displays a line on the screen, i.e., mountains versus flowers. I wound up making an estimate of focus based on the indicator on that point. Seems to me that I could apply cellophane tape to the screen, mark the points on that, and use the screen's focus indicator as my follow-focus marker.)

  • 3 Replies sorted by
  • I can't say there is or isn't something wrong with the FF, but it is definitely lens related as a whole. The auto fly by wire lenses are not consistent for focus marks because they focus at different rates depending on speed. IE you cannot easily repeat focus. It's my understanding you need a full manual lens to achieve this, though some auto lenses may play nicer, or if you practice a bunch you may be able to get a feel for that lenses tendencies. I use a voightlander 17.5mm with a trusMT FF and it is spot on accurate for extremely precise rack focusing.

  • @Brian_Siano

    Yep, it is not question about FF.

    If you search you'll quickly find that all m43 lenses that use focus by wire are unreliable if you use FF, as focus position depend on the turn speed, not focus ring position.

  • Thanks for the news. I'm assuming, then, that there's no way to have the camera handle focus-by-wire lenses as a wholly manual lens.

    For now, here are the possible solutions: 1) Use truly manual lenses. 2) When you manually focus during shooting, a horizontal line with icons depicting mountains and flowers (far and near) appears. One can make focus marks using this-- perhaps wrapping rubber bands around the screen, and positioning them for the focus points. (Too bad one can't change the firmware so that this line shows tick marks, or even actual distances.) 3) Get a big external screen that's full-definition, so one can focus by eye more effectively.