Maintaining camcorder focus for dance recitals
I own the JVC GY-HM700 with a Fujinon lens and use Varizoom rear controls for focus and zoom. The problem I've encountered shooting dance recitals is maintaining focus. The problems arise from the fact each performance starts in complete darkness. The curtain opens and the dance begins immediately. No time to preset focus for each performance. I set the back focus before the performance yet continue to encounter soft focus issues occasionally during the 2 hour performance. Any suggestions? Tried the Focus Assist feature on the camcorder as well. Think about not using the Varizoom rear focus control. Think continued use during the show is affecting back focus. Either that or my aging eyes are deceiving me! No problems when I shoot objects/people in medium or CU. Any suggestions greatly appreciated.
backfocusing is a "one time only" operation. Once made, it will stay correct for years. If your lens looses the backfocus during a single show, there must be a problem that requires service.
"Post Fata Resurgo"
First, don't use Focus-Assist while you're recording, as the result of using it will show up on your recording. Basically, when you turn on FA, it opens the iris all the way.
Also, my suggestion would be to position yourself at least 50 feet from the stage. This way, after you first focus (zoom all the way in, focus on something, then frame your shot), you shouldn't have to make too many adjustments throughout the recital.
I have done lots of dance recitals (JVC 700, Canon Lens, varizoom focus cable controller, varizoom wired zoom controller) and generally agree with Mike and Paolo. I have never worked with the Fujinon lens, but I'm assuming it's the same as the Canon as far as controls go (manual focus ring only, controllable zoom, auto exposure option). Do the back focus adjust correctly (in a controlled environment, not the day of the show. Find the correct method online if you're not sure. I made a homemade "pinwheel" chart to help me). Do it one time then generally you will never mess with it again. During filming, I have two suggestions depending on the type of filming you are doing.
1) If you generally stay wide to cover all the dancers: When you set up, using the onscreen focus assist, set the focus to the the upstage/downstage area where most of the dancers will be and don't mess with it. I do two-camera shoots for later editing. One captures the wide and one closeups. When I set the wide focus, I actually gaff tape it in place the whole show. I use my pinwheel chart to help the initial focus. DO NOT rely on the focus assist when you are in wide angle! Closeup only! Those colored highlights are useless at wide angle and are even pretty useless at mid angle!
2). Now, for the closeups (BTW for dances, my closeup is never closer than a full body height. After all it's about the dance, not the face). I do keep my hand on my focus controller and the focus assist screen turned ON the whole show. I work the focus using the FA highlight, but generally only change it when the dancers are moving upstage or downstage . When the move left to right, I usually don't mess with it (yes, left to right movement changes distance to camera, so I may nudge it, but not often). Mike is correct in that the focus assist adjustment will end up on the recording, but if you are subtle and know which direction to turn when the dancers go up/downstage, you can minimize that distraction. (I confess that most of my closeup focus adjusting gets edited out when I cut to the wide angle in a two camera shoot, but I do a lot of single camera live performances and manage it okay).
However, I disagree with Mike that the iris opens all the way using the FA (at least with the canon lens). The focus assist on my camera just changes the LCD viewfinder to BW with the edge colors. It does nothing to the lens itself. I would think it's a viewfinder option, regardless of lens (but I may be wrong here). I don't know if the fujinon lens has a focus assist selection on the lens itself, but I am talking about the focus assist button on the camera that changes the viewfinder only. The canon lens does not have any focus assist option on the lens itself.
Now, while we are on the topic of iris...Remember, with all other things equal, that the wider the iris (aperture), the more shallow the depth of field (great for cinema, sucky for unpredictable action like dance). That means when your aperture is wide open, the subject will lose focus easier from upstage to downstage. Here's where you can play with the shutter speed. Generally, I shoot with a 1/60 shutter speed to get sharper movement and let the camera set the aperture to get the correct exposure (It also lets me get some decent screen capture stills). But unless the stage is lit very brightly, my camera spends a lot of time with the aperture wide open (the onscreen display says "open"). This makes that on-the-fly focus adjustment a bit of a full time workout since the depth of field is shallow. Now if I change my shutter speed to 1/30, the camera gets more light so I don't live in wide open aperture as much. It's amazing how much more depth of field you get with just an f-stop stop or two. Of course the trade off is a little more motion blur (which ironically can give the feeling of soft focus).
Anyway, I hope some of this helps!