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Canon XH Series HDV Camcorders
Canon XH G1S / G1 (with SDI), Canon XH A1S / A1 (without SDI).


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Old April 11th, 2008, 04:06 PM   #1
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Light, zoom, AF... faulty camera or normal behaviour?

Hello again!

I'm testing my new XHA1 a lot and I have detected a strange behaviour. I was recording at 3 dB in a low light indoor situation (4 o 5 conventional 60W bulbs), with the low-light preset that I downloaded from this forum. My wife was 1.5 meters far from the camera.

- When I zoom in to the max (to her eyes), the camera loses light gradually, as expected. But when I zoom out, the light comes back "in steps", not gradually, just like little "flashes" increasing the overall light while you are zooming out.

- Also, when I zoom in to the max (trying to focus her hair), in AF mode, the image blinks a lot; it seems that it's trying to focus every single hair, the background, the skin... at the same time.

Are these normal behaviours or simptoms of a faulty camera?

Last edited by Xabier Blanco; April 11th, 2008 at 06:45 PM.
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Old April 11th, 2008, 05:10 PM   #2
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I know for one that IAF in dark situations is risky - the focus mechanism will hunt for what to focus on - especially when the conditions are less than clear, ie fog, dark areas - so you may get pulsing

I'm not sure about the zoom and light coming in steps part. Would have to try it on my own camera to be sure.

Trish
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Old April 11th, 2008, 06:44 PM   #3
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I have just recorded a "demo". Please pay attention to the "light steps" effects. It's specially noticeable in the white wall. There is also some "AF blinking" when the zoom is 100% in. The video starts after a short commercial:

http://www.hispasonic.com/videos/prueba-xha1
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Old April 11th, 2008, 06:54 PM   #4
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Trying to zoom through the range when the lens has to stop down at long focal lengths is beyond the capability of the camera. If you want to zoom in all the way you need to light to bring the image up to at least an f3.5 (I think that's as wide open as the lens gets when zoomed in most all the way). Also, auto focus doesn't work well in low light. Nothing wrong with the camera--those are limitations of the electronic lenses of any of the 1/3" chippers, unless you use one of the interchangeable lens models with a manual lens.
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Old April 11th, 2008, 07:09 PM   #5
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Old April 11th, 2008, 07:14 PM   #6
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Originally Posted by Bill Pryor View Post
Trying to zoom through the range when the lens has to stop down at long focal lengths is beyond the capability of the camera. If you want to zoom in all the way you need to light to bring the image up to at least an f3.5 (I think that's as wide open as the lens gets when zoomed in most all the way). Also, auto focus doesn't work well in low light. Nothing wrong with the camera--those are limitations of the electronic lenses of any of the 1/3" chippers, unless you use one of the interchangeable lens models with a manual lens.
Oh, ok, ok... nice to hear that :) Thank you. Anyway, that "light stops" happen when zooming out, not when zooming in. Is it normal too?
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Old April 11th, 2008, 08:26 PM   #7
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I have the answer :)

I have the answer to the brightness steps. I know exactly what this is & Canon should fix this mess. It's a luminance pulsing that is only present in A,TV, AV, & Spotlight modes while zooming out. To get rid of it while zooming out, you have to get out of a mode that uses auto iris functionality, such as using EXP LOCK in TV and AV modes or just go full manual. I only use TV mode & the EXP Lock combo.

I brought this up early last year & no one seemed to have comments much about it, or hadn't experienced this oddity. I didn't really let it bother me because I'd rather control the iris myself. A co-worker's A1 exhibits the same behavior.

addition: btw, your link isn't working.

Last edited by Bill Busby; April 11th, 2008 at 11:26 PM.
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Old April 11th, 2008, 08:37 PM   #8
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Regarding the luminance pulsing, was auto-gain enabled?

I don't use it, but wondering if it is continuously variable, or cycles through steps, 0,3,6,12 etc.?
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Old April 11th, 2008, 08:41 PM   #9
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Tom, I'm pretty sure you were addressing the original poster, but I'll chime in anyway... because I can :) I don't use autogain ever... but back when I was testing this weird thing, I noticed that when autogain is on, the brightness pulsing stops. Go figure :-\
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Old April 11th, 2008, 09:00 PM   #10
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Hi Bill,
I was actually addressing you jointly, that's why I used your term luminance pulsing to describe it. Thanks for noticing.

But what you just noted about the pulsing going away when auto gain was on does seem weird, but I wonder if there is experience to be gained from it?

It has me thinking, that if you put the shutter speed and aperture into manual, but switched on auto gain, well I guess you're exposure metering would be automatic. It's not something I'd want to do in daylight, but indoors it might have some useful application.

For example, I could set the shutter at 1/60, aperture at f1.8, switch on a camera light, and as long as I wasn't overexposing up close, the gain would compensate for light loss when zooming in (or out), apparently without luminance pulsing.
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Old April 11th, 2008, 11:40 PM   #11
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Interesting theory, but the only thing "auto" would be gain & that couldn't be pretty :) I wouldn't trust any kind of auto gain. Depending on a situation I don't go beyond 6db. If that doesn't cut it in a run & gun situation then I kick in 1/30 shutter. If that doesn't cut it, I yell... "CAN SOMEONE TURN UP THE HOUSELIGHTS PLEASE?" :)
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Old April 12th, 2008, 06:04 AM   #12
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Thank you for your answers. I don't use the auto gain mode. I'll do more tests and share them here If I discover anything new.

By the way, my link is working again. The pulsating effect is very noticeable in the white wall when zooming out:

http://www.hispasonic.com/videos/prueba-xha1
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Old April 12th, 2008, 11:46 AM   #13
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Quote:
Originally Posted by Bill Busby View Post
Interesting theory, but the only thing "auto" would be gain & that couldn't be pretty :) I wouldn't trust any kind of auto gain. Depending on a situation I don't go beyond 6db. If that doesn't cut it in a run & gun situation then I kick in 1/30 shutter. If that doesn't cut it, I yell... "CAN SOMEONE TURN UP THE HOUSELIGHTS PLEASE?" :)
I agree to the extent I've never trusted auto gain. But I've never used it. Why the mistrust...hmm...I suppose it's from reading the experience of others who complain about what? The noise!

So I understand what you are saying because that's how I am as well, never go beyond 6db, and use a light. But there are times that's going to get you dark footage. You like dark better than noise? I do too mainly but I'm going back to your observation that the luminance pulsing went away when autogain was on. Which mechanism do you think can react the fastest? Switching between shutter speeds? Switching between iris openings? <-- Both of those have mechanical speed limits. But autogain is electronic and could be orders of magnitude faster. And something was eliminating the luminance pulsing in autogain mode. What was it? Obviously it was something the autogain itself was doing.

To me it would seem that autogain itself would cause luminance pulsing as it switched between steps if it switched between discontinuous steps. But I wonder if autogain moves the gain in a continuously variable manner, like a slider?

I'm not saying the fear about autogain is unfounded, but it is the FUD about it that kept me from ever even exploring it, and the fact that I shoot almost everything in full manual anyway.

The feature I envison autogain bringing to indoor video is the option for trading underexposed dark footage for the minimum gain that would prevent underexposure.

In other words, you will sometimes shoot at 1/30 and so do I. But we both know that is its own compromise. I would be looking to shoot at 1/60th, f1.8, with a light, and having the autogain only come into play when the subject is too far from the light or the zoom causes the aperture to stop down.
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