Is negative gain the "equivalent" of low speed stock film? at DVinfo.net

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Canon XL H Series HDV Camcorders
Canon XL H1S (with SDI), Canon XL H1A (without SDI). Also XL H1.


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Old January 8th, 2007, 01:24 AM   #1
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Is negative gain the "equivalent" of low speed stock film?

should my images be richer and more saturated when i shoot a -3dB, as oppose to 0dB?
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Old January 8th, 2007, 06:31 AM   #2
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We rarely go above -3db gain. I can't remember where I read it but my understanding is that on the H1, -3 is actually 0 and that Canon set it up this way to kind of "cheat" the low light issue. Whatever the case, we have had the best results using -3 and various custom presets found here at DVInfo.
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Old January 8th, 2007, 08:51 AM   #3
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Gain control

Hi, The gain control is an electronic variable amplification of the video. It will not change the "film type" to one with larger or smaller grains with corresponding worse or better reproduction of the reality.

Remember these cameras shoot with RGB chips which only have 8 bits of resolution in the following A/D converters. Thus, each of these colors only have 256 different levels from black to the most red, green or blue color the camera can reproduce. This makes quite large steps combined with a rather small span (from the darkest black to the brightest white) so you have to use most of the span to get good results. The nature nearly always have a larger span than what you can cover with these digital camcorders. And your eye can resolve much smaller steps too.

So you have to sacrifice the deep blacks (a black man in the sunlight will look black despite he is more brown than black) or sacrifice the bright white (the sun won't look brighter than a light bulb) or aim cleverly for something in between. Often it is good to brighten the black shadows with more light.

Think about the gain knob as a gain control on a tape recorder. If you can't get close enough to a speaking person to "fill the VU-meter scale (set enough light) then you can increase the gain of the amplifier. But this will also raise the noise level by amplifying sound from other people, cars, thermal noise etc.

So better telling the actor to speak louder, moving the mic closer, keep other sound sources quiet, change to a better mic with less noise etc. Unfortunately there is not that many options with video.

By increasing the gain of the video you will probably see that the blacks are not completely black anymore. Grey or colored noise pixels flickers around, especially in the black areas where they are more visible.

Think about the relative difference. Let´s say the noise has a strength of 1 step in the 8 bit signal.

In the dark nearly black areas at RGB level = (3,3,3) a noise of (1,0,0) suddenly shifts the signal to (4,3,3), a increase of 33 percent in the red. You see a redish noise pixel in the nearly black area.

But if you have light, perhaps (50,50,50) a noise of (1,0,0) only changes the signal to (51,50,50), a relative increase of 2 percent which makes the change hardly noticable.

So, the best is to increase the light to use the lowest gain setting, no matter what the manufacturer labels it.

You will however not gain (pun intended) by staying at -3 dB when it is too dark and not get any areas exposured up to a reasonable white level. Amplification in post will also increase the noise.

I´m returning from shooting a 100 year celebration in a church and just editing now. We used 3x800 W redhead lights with softboxes in addition to the ordinary weak lights. It was just about enough to light the scene for our XL H1 at + 3dB but we had to increase to+6db in the evening. It was not enough light for our second camera, an XL2; the noise was barely acceptable.

If you can stay at gain -3dB, still have white up to zebra level, use about 1/50 s exposure and a good PRESET your video may look nearly as you should have been there with your naked eye! (Honestly, our eyes are in fact much better, sorry God.)

In a dark situation with a lot of gain and a slow shutter you can capture what is happening even if it is very dark. But it will look "video" and not reality. It is completely different but could be acceptable in certain situations.

Light is the answer, not more gain! Good luck and make your own tests!

Last edited by Johan Forssblad; January 8th, 2007 at 12:40 PM.
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Old January 8th, 2007, 08:55 AM   #4
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I agree with Johan 100%. If our image is too dark at -3, we add more light. But we stay at -3 whenever possible.
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Old January 8th, 2007, 09:09 AM   #5
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Quote:
Originally Posted by Johan Forssblad
Remember these cameras shoot with RGB chips which only have 8 bits. Each of these colors only have 256 different levels from black to the most red, green or blue color.
Johan, that's sort of true. The chips actually have more than 8 bits of resolution and are scanned as such. It's all the down sampling afterwards to fit the constraints of the format that leave you with 8 bits of level.

I'm pretty sure that even the XL2 has 12 bit resolution at the camera head before compression. I remember we kind of hashed out that question when the camera was first released.

-gb-
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Old January 8th, 2007, 11:32 AM   #6
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Agreed, but,

I gotta say, I use a 35mm adapter and sometimes with 0 or even +3 gain, it looks insanely beautiful... !
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Old January 8th, 2007, 04:09 PM   #7
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Getting to know your XL-H1

I've been grappling with this same problem as of late.. I'm hoping someone could help enlighten me. I have only had the camera for a short time, and have only shot a few non-critical things.. first was a Logo of some small objects under 1500W of soft lights.. the closest object was about 10cm from the Canon Wide angle, and the farther was about 2 feet. I cranked the Fstop to max, and everything was stunning and in focus. The second was my daughters gymnastics meet which was primarily to humiliate my daughter (god she hates it when I bring a camera ;-).

But the problem is I either get totally stunning images (i.e. 1500W shoot), or a mixed bag.. my daughters gymnastic meet.. During that, I noticed a few things, basically, when the zoom is low, almost everything was crisp and stunning.. when I zoom in (yeah.. I understand, less light).. things would loose the crispness and become far more grainy.

My problem is, it was not until I put it up in FCP that I could see this. I shot most everything at -3 also.. but when I am doing interviews under less than perfect circumstances.. I want to know that the results are going to be less than ideal so I can make a judgment call rather to proceed, attempt to modify the lighting, or scrap the shoot. I cannot detect a difference in the viewfinder, and this really bothers me.. I want to be sure of my results. How would you go about this?

I've only been trying to shoot seriously for about the last year, I'm now beginning work on a few documentaries, and I can't afford any mistakes and don't want to be forced to use substandard video.. Any suggestions are greatly appreciated. Every shoot is a learn by pain experience.. I've come a long way, but have a long way to go.

BTW: I've been employed as both a hardware & software engineer, so I have no real problems talking technical.. its just the nuts and bolts of everyday practicality and artistic vision that I fall short.

Thanks!

Brian
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Old January 8th, 2007, 04:42 PM   #8
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Hi Brian,

Did you underexposure when you shot those tele pictures? If you are working at low aperture numbers in wide angle and zoom to a long lens you get the f drop which may cause under exposure.

I always use manual mode. I put my zebra stripes at 95 % and tune the exposure to see stripes in white light parts of the picture. I usually have to sacrifice light bulbs and such intense small areas like reflections of the sun in water etc.

This will usually show a hotter than normal exposure indication at the meter in manual mode but this way I get less noise in the shadow and black areas. It is more forgiving to darken the image in post rather than increasing the levels later.

If you don't have directional light the footage could suffer and show a grey and more uninteresting image. I imagine this could be the case in a large sport hall. This combined by shooting at -3 dB could give you problem with not sufficient light for tele footage.

I would try 0, +3 dB or even +6 instead to at least "fill the meter" at wide open aperture.

Unfortunately I don't find it possible to judge the picture quality by the viewfinder. You need experience or a good portable monitor.

About the f-stop to max: Be careful, the footage will be softer at f-numbers above about 8. That's why Canon have limited the aperture to 1:9.5 in most modes.

We are nuts. Remember, film team used to have trucks with lights and electricians to connect it to not blow the fuses. Now we expect to get the same footage without any extra light ...

Please come back with facts about your exposure. I'm no expert so please chime in you other here. /Johan

Last edited by Johan Forssblad; January 9th, 2007 at 02:02 AM.
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Old January 8th, 2007, 07:08 PM   #9
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For all wildlife recording I keep my XLH1 on -3 gain as I found auto or 0 gain in low light to be too grainy. Using 1/30 of a second or even lower produces a cleaner picture for me even though it may be strobey in fast action. At any rate a slightly underexposed shot is better than a grainy one in my opinion.

Leon Lorenz
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Old January 9th, 2007, 02:00 AM   #10
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Hi Leon,
How much dare you to underexposure before you start increasing the gain?

I agree with you to not use the auto gain to know what you are doing. I have turned off all automatic functions on this camera except the OIS when not on tripod. Also the use of longer exposure time is good to have but for wildlife with a lot of life it causes unnatural effects. These could be interesting sometimes but not for all the footage in my opinion. For static shots they are great.
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Old January 9th, 2007, 03:09 PM   #11
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Thanks for the help

My settings (and lenses) were all over the place, I am in experiment mode right now.. I'm just trying to get competent with the camera in manual, observing changes between basic stuff (30F & 24F), I was mostly leaving the camera in shutter priority and playing with that as I wanted to see about where you could get fast moving images (girls in tumbles, etc) not too blury and able to see what effect it had on everything in general. I really liked the 24F on stuff not heavily in motion.. but for fast moving stuff 30F was just much nicer.. okay it doesn't look like film, but it is smooth and when the subject matter is suppose to be graceful..... well..

It's perhaps worth mentioning, kind of a funny story.. when I was trying to get a handle on some basics of the first camera I purchased about 1 year ago (Sony HVR-A1U), I would walk around film things, and talk figuring that the shotgun mike I had Sennhieser K6/ME66 would pick it up and I could listen and watch.. I did a couple of tapes over a several day period like this.. when I finally put them up in the first editor I was using (Vegas)... I couldn't hear myself.. the mics rejection was too good..or I was too quiet.. Now I keep pulling the mic off and talking into it during filming.. so I know what I was doing and the effect. I have my first interview for a documenatary in about 1 week.. its all in a controlled enviornment.. my living room, compelete with a few thousand watts in softlights.. so I think there is no problem there.. its just the ones I have to do ad-hoc in another month or so I am more worried about.

So really the only things I had in manual for this was the focus, Gain (-3), and shutter priority.. most of the speed I went though were relatively slow.. mostly around 60 - 120th of a sec.. I found that after 100 - 120.. in most places it would flash in the display. I suppose I should just start putting it in manual all the way. I haven't tried using the zebra stripes on this camera yet.. I'm going to try that.. thanks for the suggestion.

A few opinions I would be interested in to anyone who cares to answer..
(1) I bought some color cards about a year ago.. one friend tells me they are just for amatures..does anyone else use them? His comments were that if you want to make the picture have better colors do it in post.. once you've introduced something to distort the image, you can't go back. I've shot a number of things with and without... and tried tweeking in FCP, but I think if I was doing something I didn't want to waste alot of time in post with, I may use them.. i.e. if I was doing a wedding or something where I didn't consider it so much a work of art as a work of commerce.

(2) Anyone ever used external light meters? Is it worth the investment?


I've probably a hundred other questions, but I won't push my luck as I am very thankful for this resource ;-)

Thanks!

Brian
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Old January 9th, 2007, 03:14 PM   #12
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OIS question/comment

I noticed with a Sony HVR-A1U that I had that I had alot of weird stuff happen that I attribute (I could be wrong) to the stabilization.. Specifically, when I was filming in a more urban setting, close to buildings.. it would get some strange jerky movements on the straight edges of buildings, windows, door frames, etc as I would walk by. I know that was electronic and this is optical, but should I expect the same kind of results (with optical stabalization of the XL-H1)?

Thanks,

Brian
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Old January 9th, 2007, 09:27 PM   #13
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Hi Johan

I find 1/30 to have less grain than adding gain in most shots. Of course in fast action it will be strobey and gain would be a better choice. Unless the shot is rare I seldom keep these less than perfect shots in wildlife filming.

Happy shooting,

Leon Lorenz,
www.wildlifevideos.ca
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Old May 18th, 2007, 10:25 PM   #14
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Gain Setting

I agree with Leon . . .and I also shoot entirely outdoors / wildlife. Several months ago when I had just purchased the XL-H1 I did a shoot in Central America. Of the 12 hours + of footage some was very grainy. BTW - this wasn't apparent in the EVF or LCD. The grain shows only on a monitor. I had the gain set at 0 during the entire shoot and as a result some of the footage is unusable.

After restting the gain to -3 I don't seem to have any problems with graininess.
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