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Tim Tyler

Backfocus Adjustment

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How is the backfocus adjusted on the DVX100a?

 

I just read that on the XL2 is software controlable by a service rep.

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Sometimes the VX100a can go out of focus. Often the result of being dropped. I found that powering down and pulling the battery will reset this problem.

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Sometimes the VX100a can go out of focus.  Often the result of being dropped.

 

Haha, I'm guessing that's if you're lucky! ;)

 

 

 

Hmm... now that I think about this, every time I power up my camera (not a DVX), I notice that it zooms in, the focus goes way out of whack, then it returns to where it was. Is this checking the lens backfocus, or simply 'initiating' the focus/zoom servo motors, or something else?

Edited by Alvin Pingol

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Is this checking the lens backfocus, or ...

 

I don't know.

 

But I Googled around and discovered that others have experienced back focus anomalies with the DVX100 too. Basically - if you zoom in, focus, and zoom out, the wide is soft :(

 

The camera I noticed it with had a filter-thread-mounted Chroziel matte box, so I'm wondering if maybe the added weight stresses the lens assembly.

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Basically - if you zoom in, focus, and zoom out, the wide is soft :(

 

Well, from what I understand, that little technique to get the smallest COC never worked with this camera, since it does something different with the "numbered" focus adjustment. Hence the DVX focus charts.

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Hence the DVX focus charts.

 

Can't find those anywhere online now. The old link is dead.

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Speaking of odd DVX-100 focus quirks, my camera has exhibited a particular focus issue from the day I got it, but I never really had a chance or the time to take it in and get it checked. I've tested various DVX-100's at trade shows and some exhibit the same problem, others don't.

 

The problem is this: When doing manual snap zooms in or out, the focused-on subject goes out of focus during the zoom and snaps back into focus at the end of the zoom. Not the usual motion blurring of the image's outer edges that occurs during a snap zoom, but a definite defocusing of the very center of the image.

 

Very strange and annoying, since I'm particularly fond of snap zooms both for stylized narrative work and for doc work.

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Hi,

 

The only thing I can think of which might explain this as a standard "feature" is the frequently-used trick on small handycams of not actually bothering to build a trackable zoom. That is, focus is maintained during the zoom by the software's knowledge of the zoom position and the optical result that's hitting the CCD, and continuously tweaking the focus to keep things in. This makes it a lot cheaper to build the lens - you don't really have to care much about the positioning of the lenses so long as they're coaxial, the software just sorts it out - but it does mean that the focus group will not be in the same configuration for 10' focus at the wide end as it will for 10' focus at the long end. This is why they don't do real manual focus controls - you simply can't, since the position of the thing needs to change all the time as you work with the zoom.

 

This trick generally works OK, and at least on the DVX they've encoded a 1:1 relationship between the focus distance and the position of the ring. However, it does mean that the zoom will track focus only as fast as the servos can manage, and if the zoom servos are faster than the focus servos, then you will see exactly what you're seeing.

 

I don't know that this is true, but it's a good theory which fits several available facts - bad tracking during zooms, no manual focus ring on the lens, handycam.

 

Phil

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and at least on the DVX they've encoded a 1:1 relationship between the focus distance and the position of the ring.

 

Thanks, Phil.

 

I'll mess with it this weekend with that in mind.

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Dear All,

 

So, my trusty DVX100Ae needs it back-focus adjusting. Does anyone know roughly how much the service will cost to fix the back-focus? Just looking for a ball-park figure. I'm based in London, UK.

 

Many thanks,

Jack

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Hi Tim,

 

Most of the fixed lens camera do not have a real zoom lens, but a varifocal lens. It's cheaper build & breathing can be reduced. There is an engineering set up that will auto set up the zoom when the camera is pointed at a focus chart. The focus has to be calibrated at different focal lengths this data is used to make the lens track in focus.

 

Best

 

Stephen

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