Little Help? [Low Light Focus Problems] at DVinfo.net

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Canon GL Series DV Camcorders
Canon GL2, GL1 and PAL versions XM2, XM1.


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Old March 28th, 2004, 09:16 AM   #1
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Little Help? [Low Light Focus Problems]

Thanks for the help in my previous question. I ended up at my first shoot Friday, but it didn't turn out as well as I had hoped. I was so concerned with light, I forgot that auto-focus in a low-light is a no-no. Anyhow, after 20 minutes of shooting I put my thinking cap on and turned MF on. However, my troubles did not end there. As a note, I had the WD-58H wide angle adapter on. Anyhow, I was under the impression that zooming in tight and focusing and then zooming out for a wide shot was the correct procedure? I guess I was wrong, all my wide shots are slightly off focus. I can't really tell what plane is in focus, and truthfully I really couldn't tell in the viewfinder at the time. I think I was more concerned with framing etc.

I encoded a small clip to illustrate what I'm talking about. Does anyone have a depth of field chart for the wd-58h? Should I open the iris up and turn the shutter down to 1/30 for a greater depth-of-field?

Thanks for any advice. I understand there's a learning curve on this camera, but I want to be better prepared for my next shoot in about 2 weeks.

http://chief.fishbucket.com/crap/pernicetest.wmv
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Old March 28th, 2004, 09:57 AM   #2
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I also use the WD58 and I've had to go with auto focus in low-light many times and only rarely does the camera briefly lose focus...but it can happen. I've found that when I go MF, if I zoom to the subject or there abouts, focus and then go wide, everthing stays in focus. I believe, moving the camera to a different subject and zooming in will require another re-focus. It seems for me that whenever the camera is at wide angle, it's always in focus, manual or auto. Correct me if I'm wrong but that's what I remember from last years' events.
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Old March 28th, 2004, 10:08 AM   #3
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The standard procedure is to zoom in, focus and then reset frame size as required. It should not lose focus when you zoom out.
My WD-58H wide angle adapter virtually lives on the camera, with no problems, so it can't be related to that. I can't think for the life of me why it lost focus.

Robin.
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Old March 28th, 2004, 11:10 AM   #4
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Just a thought -- isn't it possible that in low light you didn't actually set the camera in sharp focus when you zoomed in? (Maybe you were too close to the musicians.) You said yourself that you were concentrating mostly on framing, etc. and couldn't really verify the focus in the viewfinder at the time.
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Old March 28th, 2004, 11:23 AM   #5
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One other thought. I would'nt rely on the viewfinder for absolute focus. Use the LCD...what you see is what you get.
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Old March 28th, 2004, 11:59 AM   #6
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<<<-- Originally posted by Jim OMalley : Just a thought -- isn't it possible that in low light you didn't actually set the camera in sharp focus when you zoomed in? (Maybe you were too close to the musicians.) You said yourself that you were concentrating mostly on framing, etc. and couldn't really verify the focus in the viewfinder at the time. -->>>

Well, as you can see from my clip, i zoomed in and the singer is in focus, i zoom out and BLUR, and I didn't touch the focus ring on zoom-out. As for being too close, I was about 15-20ft away during the sample clip.

RE: viewfinder vs. LCD.. why should this make a difference? I suppose I'll use the LCD next time, but shouldn't things appear correctly in the viewfinder?

One more question: when doing close shots (0-5ft from subject), is it preferable to take off the wide-angle? Will I get a better depth of field in my closeups with the standard lens?


Thanks!
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Old March 28th, 2004, 02:17 PM   #7
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The viewfinder can go out of focus from handling and it could give you a false reading. That can't happen with the LCD.
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Old March 28th, 2004, 02:18 PM   #8
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This is a common complaint. There are two possible causes.

1. Your camera's zoom lens has lost its back-focus settings. This is the less likely cause with cameras such as the GL2/XM2 that feature integrated zoom lenses .

2. More probably the lens' iris automatically closed a stop or so in the zoom range, thereby altering your depth of focus. That is, you may have composed your shot and set your exposure (iris) for this wider shot. You then pushed the zoom lens close to your subject to snap manual focus, then pulled it back to recompose your shot. You may not have noticed that your iris actually closed down a stop or so as you pushed the zoom to snap focus. This is normal, indeed essential, behavior for the camera. When you pulled the zoom back you probably reset the exposure (iris) thereby altering the focal field and invalidating your focus adjustment. (Remember, focus with a zoom lens is a function of the lens' current internal optical configuration and the iris' setting.)

This tends to be more of an issue in darker scenes that require the iris to be nearly wide-open to get a decent exposure. This makes for a shallower depth of focus for the wide shot but a slightly deeper depth of focus for the tight shot used for focus.

One solution is to make sure that you do not push the zoom beyond the iris setting that you will use for the actual shot. This may mean that it will be more challenging to snap your manual focus, since you cannot get as close as possible.

You might also try snapping your focus (without altering lens zoom) by pressing the Manual Focus button to engage auto-focus for a moment, then again to restore Manual Focus. The effectiveness of this depends on the amount of light and contrast in the scene.

You can easily experiment with this to see for yourself.

As veteran camera pro, and long-time fellow DVInfo member, Wayne Orr used to say in his signature, "If it was easy, they'd have a relative do it."

Have fun!
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Old March 29th, 2004, 08:55 AM   #9
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Thanks so much for the advice!

Ken, I tend to agree with your description. Though I could've swore I had the camera in manual mode, with about f1.8 & 1/60 locked in. I think I will get out onto the streets this week and do some experimenting. Hopefully I can become a GL-2 guru in the next 2 weeks.

Thanks again, this board is a great resource!
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Old March 29th, 2004, 12:11 PM   #10
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Christopher,
I've no doubt that you had the camera's exposure dialed-in manually. That's what can make this particular phenomenon so perplexing, particularly to new shooters. The iris must still close-down a stop or so when you push the zoom all the way out, even when you're in 'M" exposure mode.

Now that you know this, keep your eye on the iris' setting on your LCD/EVF when snapping critical focus.

Can you tell that this has nailed me on more than one occasion...with more than one camera? <g>
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Old March 29th, 2004, 04:22 PM   #11
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Ken, thanks.

Actually, I just reviewed my footage on a TV for the first time, and I was pleasantly surprised. I guess the pixel ratio of my monitor made things seem blurrier than they actually were. My wide shots are quite quite "flat" but they aren't necessarily out of focus. So: phew. I guess I should probably set up a TV monitor next to my computer monitor for color syncing etc. Anyhow, the club I shot in was probably some of the worst lighting I've ever seen, I'm sure that the venues in bigger cities (NYC, Boston) will have far better stage lighting (well, here's to hoping, right?)

I will definitely still go out and practice, and keep an eye on my stops!

Thanks again!
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