HV20 image suddenly overexposed with no lighting change? at DVinfo.net

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Canon VIXIA Series AVCHD and HDV Camcorders
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Old January 28th, 2008, 09:29 AM   #1
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HV20 image suddenly overexposed with no lighting change?

I've been using an HV20 for months with great results.

I use fluorescent lights and the same camera position. I've changed nothing that I'm aware of.

Suddenly, the image on the LCD screen, and the recorded image, is way overexposed. People look like white vampires.

If I turn the lights down (I can turn off the lights in banks), nothing changes, they still look overexposed.

It is as if the camera's light sensor is getting the wrong readings. I've tried zooming in so that just a human face is all the camera sees, but no change.

I can have the 'talent' hold up a white sheet, and that drops the exposure on their faces noticeably - so the camera could stop the exposure down if it wanted it.

What obvious thing am I overlooking to return things to normal - or should I create a Goth film?
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Old January 28th, 2008, 11:28 AM   #2
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Got me curious on this one.... When ever I start mine..... even if it was locked in an exposure setting like TV or AV, once it is turned off, it goes back a semi auto mode, till you engage toggle.... ND filters jammed in open mode ?? Are you in Custom mode in which you ramped up exposure, or is one of the electronic effects engaged too, causing the issue ??

Let us know what you find out...
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Old January 28th, 2008, 12:34 PM   #3
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Quote:
Originally Posted by Les Caudle View Post
I've been using an HV20 for months with great results.

I use fluorescent lights and the same camera position. I've changed nothing that I'm aware of.

Suddenly, the image on the LCD screen, and the recorded image, is way overexposed. People look like white vampires.
Are these modern fluorescent lights with a high frequency ballast, or much older fluorescent lights with a 60 Hz ballast? If the latter, that could be the problem, as there could be a strobing effect which could mess up light readings.
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Old January 31st, 2008, 07:19 PM   #4
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David - these are new fluorescents from imageWest.tv that should not flicker.

Chris - after reviewing 'good' footage frame by frame, I can see where the problem occurs.

When I have one person in the frame (with an evenly lit green screen behind the camera with its own lights), the green screen looks 'fluorescent' and the person in front of the camera looks overexposed.

When a second person enters the frame, so that the frame is mostly two heads and upper torsos, the green screen behind the camera instantly looks 'normal' and both subjects look great.

The HV20 seems unable to figure out how to set the exposure unless there is more than one person in the frame.

And now, I've messed with the settings so much that I have no idea where I started from.

I bought a Gretag Macbeth colorChecker white balance card, and when I set the white balance using this card, everything looks way too warm, not correct at all.

So - I'm thoroughly confused as to how to proceed.

I guess I'll attempt to lock the exposure, and adjust it down until the subject is properly exposed.

Then - see if I can get the white balance back to where I had it (I forget if set to daylight [these are supposed to be daylight fluorescents] or to fluorescents.

Any clue why my attempts to white balance have failed so miserably?

Or even if I accidentally had the auto/P slider in the auto position.

It had just looked so great before, never been this frustrated!!!

Maybe there is a way to tell the camera where to look to set the exposure (like on my Canon 300D)?
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Old January 31st, 2008, 08:26 PM   #5
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Just got off phone with Canon tech support.

My tech spoke to another higher up tech, who said that the HV20 uses the full screen to set exposure (and focus?) and has problems with a single person in the shot - as it is take the exposure of the entire viewing area.

This certainly matches exactly my problem.

I hope that they will introduce a flash update for this - as this seems a glaring defect (at least to me). Am I missing something obvious here?

My 3 year old Canon 300D (bought at same price point) has the ability to set the area on the screen to use for auto focus and exposure. I'm a bit frustrated!

I'll try locking the exposure and dialing it down (and thus bring up the f-stop while in Tv mode) and see if I can get decent results.

What is the max f-stop I can use before I get a degraded image?

Same question for the shutter speed (I think I read in the manual it has trouble focusing at 'high' shutter speeds - but the manual didn't state what 'high' was)?
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Old January 31st, 2008, 09:33 PM   #6
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Les:

Are you shooting this in automode ?

Top slider switch on the tape door must be set to P, so you can do manual adjustments. You need to turn that off, especially for green screen.

In addition, its possible you may have engaged back light function on the camera, and don't realize it. Thats the button just above the focus roller.
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Old February 1st, 2008, 08:16 AM   #7
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Sounds to me like an auto exposure mode artifact. Scenes with unusual lighting or white balance can fool teh corresponding automatic functions in camcorders, call for use of manual or locked down exposure and white balance settings
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Old February 1st, 2008, 10:20 AM   #8
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Chris - I'd checked to make sure the back light was off and I'm in P mode.

Don - yes, I got it to work and look good by locking the exposure in Tv mode and increasing the f-stop (so no worries about adding gain) - and did custom white balance using a Gretag Macbeth color checker card.

I guess I'll have to measure exposure using the scopes with FCP in histogram mode (I capture directly thru HDMI).

Sure hope they 'correct' this and add the same capabilities my 300D has in regards to exposure - as even without a green screen, a single face can look very overexposed.
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Old February 1st, 2008, 12:43 PM   #9
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Les:

I have just started some green screen work with this camera also. I made my own screen with canvas and Sparkling Apple green paint from Home Depot.

I lit the screen with fluorescents, and lit subjects separately with incadescent. Not sure if that is considered kosher, but I was trying to avoid flipping breakers. I actually shot my first test by turning on the camera, which is set for 24p, Cine mode, using white balance set for incandescent bulbs, and let the exposure adjust. I walked into picture. I was amazed at how well the camera shot the scene and how easily it keyed for a novice like me in Vegas.


Not sure why you are having so much trouble, but it is beginning to sound like a camera issue, especially since you seem to imply it worked before under the same set up. Digital affects on ??
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Old February 1st, 2008, 01:03 PM   #10
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Chris - I've got filters on my green screen lights

http://dvinfo.net/conf/showthread.php?p=784330

It may be that the concentrated green channel confuses the camera, but in some testing with just one light on me and no green screen, it was overexposed much of the time.

I don't think the camera is happy with one subject in front, it needs more subjects (taking up screen space) to calculate exposure correctly - which would easily be fixed by some settings to tell it which areas to monitor for focus/exposure.
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Old February 1st, 2008, 03:00 PM   #11
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Auto exposure and camera metering can be confused or fooled by scenes with dominant lighting components. That is why higher end digital cameras provide metering alternatives such as average, center weighted, and spot, and exposure locking features as well as manual exposure capability. Of course, having these alternatives modes available adds cost, and also separates the professional and prosumer gear from the consumer stuff.

We cannot expect to get away with point-and-shoot methods that work under typical average conditions when we get into unusual situations, like green screen, or one modest sized object on a large area of dark or bright background.

While some of the auto features like back-light correction may help, there is no reliable substitute for using manual exposure and white balance, and a good monitor to evaluate the settings, if the final image exposure and color product is important, as it would be for green screen work..
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