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Zooming too abrupt, during video recording -- unable to hold focus

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Offline RonCam

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Zooming too abrupt, during video recording -- unable to hold focus
« on: 25 / August / 2008, 06:03:58 »
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When I recently tested the new capability of CHDK to enable zoom during video (a welcome addition, thanks!) I discovered a problem.

The Zoom works so rapidly that the camera loses control of focus and exposure.  In addition, the effect of an extremely rapid video zoom is visually disturbing. 

This particular zoom scene went from telephoto to wide angle.  Exposure and focus were was good before starting the zoom.  Everything sort of "went to pieces" during the zoom.  Once the camera reached the wide-angle setting, however, the image settled down to correct settings, after perhaps a half-second.

My feeling is that there's really no practical use for such a rapid zoom speed, during video recording.

Has anyone else noticed this, or found a work-around?
« Last Edit: 25 / August / 2008, 11:56:27 by RonCam »
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Offline LjL

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Re: Zooming too abrupt, during video recording -- unable to hold focus or exposure
« Reply #1 on: 25 / August / 2008, 07:19:12 »
I don't think there is a work-around. The zoom motor works in (not very many, 14 on my camera) discrete steps, so the best you could do is zoom one step at a time, and that would still be not very pleasing.

Focus is only adjusted by the camera after you've finished zooming, and I suspect that's quite an intrinsic behavior, too.

You could, possibly, zoom in small discrete steps and then use something like Deshaker to smooth things out.
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Offline RonCam

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Re: Zooming too abrupt, during video recording -- unable to hold focus or exposure
« Reply #2 on: 25 / August / 2008, 07:44:14 »
Now I know why Canon disabled the zoom in video mode.

If you could see the result, it would take a lot more than Deshaker to repair it.  But, thanks for the link.  Deshaker sound like a very good utility for other purposes.

Given what you said about the hardware limitations, then the best workaround is to get the zoom over with, as quickly as possible, and edit it out.  Having said that, I'm hoping Canon's video editor (in ZoomBrowser) allows maintaining the audio stream while inserting a transition shot.  If not, I'm back where I started. 

The real reason for trying this was to maintain uninterrupted audio while changing the focal length.  Canon's solution would be to stop the recording (audio too, of course), reset the focal length, and resume.  This's what I was hoping to avoid with CHDK.

Thanks!
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P.S.: If nothing else is possible, to simulate a conventional, smooth zoom, perhaps zooming in discrete jumps, with enough of a delay for the focus and exposure to catch up, would be (much) better that what happened with allowing the zoom to just go ahead, at normal speed.

We could call this new kind of zoom a "special feature" ...  :D
« Last Edit: 25 / August / 2008, 11:57:34 by RonCam »
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Offline LjL

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Re: Zooming too abrupt, during video recording -- unable to hold focus or exposure
« Reply #3 on: 25 / August / 2008, 07:51:53 »
Well, in any case, if you keep the audio during zooming, you end up hearing "some slight" noise...
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Offline RonCam

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Re: Zooming too abrupt, during video recording -- unable to hold focus or exposure
« Reply #4 on: 25 / August / 2008, 08:04:43 »
Quote from: RonCam on 25 / August / 2008, 07:44:14
If nothing else is possible, to simulate a conventional, smooth zoom, perhaps zooming in discrete jumps, with enough of a delay for the focus and exposure to catch up, would be (much) better that what happened with allowing the zoom to just go ahead, at normal speed.

We could call this new kind of zoom a "special feature" ...  :D

Sorry, I should have written a new message instead of editing with my afterthoughts.  You are so well-informed on the "insides" of the camera, that I want to run this past you, to see if this is a possibility.

Yes, there may well have been some motor noise during the zoom, but to tell the truth, with what was going on with the image being so distracting, I didn't notice it ...
« Last Edit: 25 / August / 2008, 11:58:33 by RonCam »
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Offline LjL

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Re: Zooming too abrupt, during video recording -- unable to hold focus or exposure
« Reply #5 on: 25 / August / 2008, 08:30:16 »
You certainly can do that, although who knows how decent the results would look like.

Perhaps it's possible with a script, but if it isn't, it's still very possible to do in the C code (I don't have a C compiler available right now, though). Then I'd still use Deshaker to smooth out the results.

But the defocusing effect will certainly still happen... it will just be less apparent, hopefully. But it won't go away.

I must say, though, that my (old) camcorder has the very same problem... and we still used it ;)


While you don't have the C code, you can still get an idea of the sort of results you're likely to obtain by just hitting the zoom knob very briefly several times, and then processing with Deshaker. Although I suspect that doing it in the CHDK code might allow slightly more frequent zoom steps.

Play with the Deshaker settings, don't use the defaults (which are intended for image stabilization, not zoom smoothening).
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Offline RonCam

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Re: Zooming too abrupt, during video recording -- unable to hold focus or exposure
« Reply #6 on: 25 / August / 2008, 08:51:18 »
Quote from: LjL on 25 / August / 2008, 08:30:16

But the defocusing effect will certainly still happen... it will just be less apparent, hopefully. But it won't go away.


Thanks, very good suggestions to smooth out the zoom.  But I may have over-emphasized the "visual disturbance" of a rapid zoom.  A even zoom is nice, but when viewing the "footage," having the exposure and focus bouncing about, is worse.  Am I correct, that Deshaker would be of little help in this area?

Would it be possible with a script to: detect the beginning of a zoom (press of zoom lever); lock AF; lock autoexposure; detect completion of zoom (release of zoom lever); unlock AF; unlock autoexposure?

« Last Edit: 25 / August / 2008, 12:00:42 by RonCam »
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Offline LjL

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Re: Zooming too abrupt, during video recording -- unable to hold focus or exposure
« Reply #7 on: 25 / August / 2008, 09:03:40 »
You're correct, Deshaker doesn't help with that. What helps with that is zooming in small discrete steps and then refocusing after each step (which, in turns, makes the zooming effect terrible, but that's where Deshaker helps!).

No, it wouldn't be possible to lock AF, because AF is locked during zooming. The problem is that after you change the focal length, the lens needs to be refocused to match it, because the focus point changes when you change the focal length.

So what you want is almost the opposite of keeping AF locked... you'd like AF to continuously adjust the focus while you're zooming. I don't believe that's possible.
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Offline RonCam

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Re: Zooming too abrupt, during video recording -- unable to hold focus or exposure
« Reply #8 on: 25 / August / 2008, 09:31:49 »
Again, good answer.  I'll now have to work with the camera some more, to see if the problem may be minimized.  Smaller zooms perhaps?  Or, better in the end, just forget about zooming!  If any other questions, will repost.  You have been very helpful.  :)
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Offline RonCam

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Re: Zooming too abrupt, during video recording -- unable to hold focus
« Reply #9 on: 25 / August / 2008, 12:34:06 »
To avoid misleading those searching the forum, I removed or struck-out my references to an "exposure" change.  There was none.  In looking at the original scene in slow motion, I see the sudden brightening of the frame was due to a large expanse of white wall becoming included in the field of view after the shift to wide-angle.  So that part was in error.

Also, the extent of the focus problem was due to low lighting levels in the original scene.  In bright daylight, the focus change appears to be much less.

Will check to see if there are any other problems.
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