Exposure Indicator - Presets - Zebras - and being dim!? at DVinfo.net

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Canon GL Series DV Camcorders
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Old August 3rd, 2005, 08:03 AM   #1
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Exposure Indicator - Presets - Zebras - and being dim!?

The being dim part will come "to light"

I've had XM2s since Nov '02. The camera keeps on amazing me. I learn something new each time I use it.

Here's the thing:

1/- Set to Manual

2/- I can see the "overall" exposure scale - excellent!

3/- I created and use a Custom Preset that "boosts" light levels

4/- I apply the CP and observe the light levels rise in the view finder - excellent

5/- This rise is NOT, repeat NOT, reflected in the overall exposure scale. The reasoning of this I can kinda grasp. After all I'm "setting" the cameras ability to UP exposure, and consequently the over exposure scale might not logically be expected to "swing" to the right. I can kinda buy this.

. . .BUT!

6/- Then why, oh WHY do the ZEBRA bars get more numerous when I apply my LIGHT BOOST Custom Preset? HMMMMM?!?!??!

If the Zebras bars are indicating an increase in light, then why isn't the exposure scale too?

Here's the thing: If this IS the case then it is a short step for me to conclude that the Exposure Scale is somehow NOT in the same exposure "information" loop as the Zebras. Yes? BUT when I apply a BIGGER iris both the Exposure Scale AND the Zebras are effected. So it would appear that the CP is NOT connected to the exposure Scale, BUT the Iris & Zebras are.

This has really confused me .. and for my money, this "non-connection" it is NOT a little silly too.

So, Newbies, if there are any left for the XM2, be aware of this.

.. I bet somebody is now gonna quote a page of the Manual at me . ..

Best regards,

Grazie
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Old September 23rd, 2005, 06:40 PM   #2
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I'm surprise no one answered this... It's rather simple really.

When light enters the lens, Exposure controls (Shutter, Aperture and Gain) affect the level of exposure: Aperture controls how much light hits the CCDs, shutter affects for how long the light level is "read", and gain boosts the electric signal off the CCD. The exposure scale reflects the average brightness of the image at this point.

Custom presets (colour gain, colour phase, sharpness and setup) are applied to the electronic signal after this. In your case, if you custom presets raise the overall brightness of the image, I imagine that you raised the setup level. (Gain controls saturation, phase the hue, and sharpness is obvious)

Zebra patterns reflects the luma level of the final image (after the presets are applied). It tells you which parts of the image are above the luma threshold that you set in the menu (80 to 100).

In all the following cases will the zebra bars get more numerous:
- opening up the aperture (more light hits the CCD)
- decreasing shutter speed
- raising the gain
- raising the setup level

You've probably figured out by now why the exposure scale does not react to preset adjusments: it's an "exposure" scale after all, it isn't affected by what you decide to do to the image after the image has been "exposed".
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Old September 24th, 2005, 02:10 AM   #3
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Jean-Francois, thank you for your wish to clarify these points for me.

Ok, here goes . . .

I was not aware that Presets do something to an image "after" it is affected by the other options. First misconception of mine. I had imagined that a Preset was that. I was "PreSetting" a set of parameters PRIOR to making the other exp, shutter etc etc adjustments. I was making a global presetting using a PRESET. And yes it was my observation that the picture got brighter when, as you observed, "In your case, if you custom presets raise the overall brightness of the image, I imagine that you raised the setup level." - this was exactly my consternation.

My second misconception was having the word "GAIN" within the Preset "Colour Gain", and, outside the Preset settings, Gain being used to raise the luminance of the picture - this in turn affecting zebras and exposure scale. To me this "implied" a causal and definition link. Both "Gain" and "Colour Gain", in my mind, should affect the exposure scale. Added to which the electronic Gain IS something that is done to the picture AFTER it has "fallen" or arrived at the CCDs - yes? The brightness GAIN is an amplification of the signal? Yes?

My third misconception, triggered by my observation that now I can "affect" the number of Zebras using Presets AND other settings, was my conclusion - why isn't the exposure scale being affected too?

"You've probably figured out by now why the exposure scale does not react to preset adjusments: ", well, not really. The "why" would mean me seeing and observing the various amplification signal pathways. I suppose somebody could draw me a circuit. However, I am resigned to accepting your explanation, "it's an "exposure" scale after all, it isn't affected by what you decide to do to the image after the image has been "exposed". " - But even within your explanation is the hint of the following: Isn't "gain" exactly that? Isn't gain (6db, 12db and 18db), colour gain and setup everything that happens to the image after it falls on the CCDs? Am I to accept that presets - which appear to do something akin to the GAIN - have been purposefully removed from the exposure scale loop to give the appearance of separation? To give me an illusion of separation?

Fundamentally, gain is an amplification and the presets are adjustments to the signal too. I wish somebody could tell me that the Preset circuitry has been excluded from the exposure scale response. And maybe, just maybe, the presetting thing is "presented" to us in this format so to give us the option to ascertain the non-effect of presetting BUT its effect on Zebras.

J-F? Please don't read my thoughts on the non-existence of manual white balance - it will drive you nutz!

Jean-Francois, again, thank you for your kind and patient response.

Best regards,

Graham
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Old September 24th, 2005, 10:59 AM   #4
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Graham,

I think you have to see the custom presets (colour gain, colour phase, sharpness and setup) as secondary image controls. They are equivalent to effects you would apply in your NLE to transform the image. The only reason they are called PRE-set is because you can save and reload them. Otherwise they'd be called something else I guess.

Colour gain is just a colour saturation control: turn it down and the image becomes almost black and white; turn it up and the colours are more intense. But it does not affect the luminosity of the image. It has nothing to do with the Gain control you find under shutter and aperture controls.

Color phase is a hue control: it changes the tint of the image. You could use it to offset the white balance result. This does not affect the luminosity either.

The setup level, I believe is a bit of a misnomer. It looks like a gamma adjusment to me. But I could be very wrong: I'm not a video engineer after all. It clearly does affect the overall luminosity of the image however.

As for Exposure controls, the Gain is indeed an amplification of the signal after it has been read by the CCD. That gain is considered an exposure control rather than a secondary control might seem arbitrary, but I think that's for practical reasons. Gain is after all very useful in low light situations, though it makes video noise more apparent. To have it hidden inside a menu would be impractical. This is unlike the presets, which you probably won't change from shot to shot.

The exposure scale is just a reference. It lets you know where your primary settings stand with respect to the camcorder's own reading of the scene. Why show the impact of gain, but not setup level? It might have just been confusing, as the setup control does not show on the screen with the aperture, shutter and gain. Most people will leave setup at default or close anyway.

The reason why zebra patterns reflect the brightness level at the end of the chain is this: when set to 100, they are meant to tell you which areas of the image are going to blow out on the taped image. You can't recover details from blown out areas (it's all 100% maximum white). Zebra patterns serve as a warning: if you see them, you should readjust (it might be ok to have them in a very small area of the image though). Zebras wouldn't be of much use if they ignored settings that could potentially ruin your image. Broadcast video engineers will physically hurt you if you submit a video with substantial blown out areas.

When zebras are set to a lower value, say 85, they can be used more as an exposure guide: for instance you might light and expose your subject such that his face has just a bit of 85-zebra bars on the hightlights.

From my understanding (and guesses), the overall light-signal pathway is this:
- Light hits the lens
- Iris opening (aperture) lets some light in. The bigger the opening (f1.6 at the max) the more light is let through.
- Light hits light sensitive CCDs
- Shutter speed determines for how long the signal from the CCD reading is recorded, affecting brightness and motion rendition.
- Gain is applied to amplify the signal
- Exposure scale position is computed here
- White balance setting applied here (this is a guess!)
- Custom presets are applied to the image
- Zebra patterns are computed
- Image is compressed by the DV encoder, and recorded to tape

Now, a lot more processing occurs in this pathway. For instance, when is colour-space information dropped to go from a 4:4:4 signal to a 4:1:1 one (for NTSC)? My guess would be before compression to DV, but I could be wrong.

Also, the custom presets don't do anything that you couldn't do in your NLE (the same could probably be said about the gain...). BUT, since they do it to a non-compressed signal, it might be better to get as close as possible to your intended "look" in camera. On the other hand, I know a lot of people who prefer to record a flatter image (slightly under-exposed, lower saturation, low contrast), and correct it on the computer: the idea being that if you set the settings too far, you can't recover from it.
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Old September 24th, 2005, 12:24 PM   #5
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Jean-Francois - wow! So I didn't scare you off? Thanks ..

What does this demonstrate? A little knowledge IS a dangerous thing. But with your "illumination" and instruction I have "gain-ed".

Seriously, thank you very much.

G
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Old September 24th, 2005, 12:54 PM   #6
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These explanations were highly enlightening for me.
Thank you very much.

James Bright
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Old September 24th, 2005, 01:10 PM   #7
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Jean-Francois

You should be immediately promoted from "New Boot" to "Inner Circle"!!

Thanks for your insight.
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