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-   -   24p questions (https://www.dvinfo.net/forum/canon-xl-gl-series-dv-camcorders/34265-24p-questions.html)

Jeff Donald November 4th, 2004 08:03 PM

International Standards Organization (ISO), the US formerly used the ASA (American Standards Association) designation.

Video chips don't respond in a linear fashion. This has been discussed extensively in the past. You might want to do a search and save yourself some time and effort.

David Lach November 4th, 2004 10:39 PM

Thanks for the info, but really I wasn't looking to get pin point accurate readings out of my light meter. I understand it cannot be used as precisely as with film.

I guess I won't post results anyhow since it seems the ISO equivalent is very on-camera settings related, but I'm still willing to find a rough number for my particular settings.

This is video, I'm still adjusting, and understand I will need to rely much more on the waveform monitor and zebra pattern, which are pretty much new tools for me, but I will still heavily use a light meter to set my contrast ratios. I will also try to get a global reading for exposure. I'm not even talking F-stops here, just general guidelines like not enough light for exposure without using the gain up, deep or shallow DOF, too dark, too bright, etc.

I'm comfortable with a meter and even though I know videographers don't find much use for it, I will still feel more comfortable getting a rough lighting setup with the light meter and then fine tuning with the zebra and waveform monitor.

Rainer Hoffmann November 5th, 2004 02:44 AM

<<<-- Originally posted by Jeff Donald : Video chips don't respond in a linear fashion. -->>>

Ah, Jeff, i don't agree. CCDs or CMOS chips do respond in a linear fashion. When you double the brightness (the number of photons that is) the output voltage of the CCD or CMOS element doubles also.

That is a major difference between digital photography and "film" photography. Film as well as our eye responds in a logarithmic fashion. Twice the amount of light does not seem twice as bright to our eye.

Christopher Go November 5th, 2004 05:29 AM

Found this thread and a post from the always knowledgeable Don Berube about this subject:

Quote:

During training, we were informed that when in 60i, 16:9 Mode, 1/60th second shutter speed, 0db Gain that the effective ASA is 400. Turn the Gain knob to the left and you will have 320ASA at -3db. Was easily capable of producing a bright exposure even at 1/100 second inside the Javitz Center.

In 24P at default 1/48th second shutter speed, it is then of course very good.
(From early reports at DV Expo on the XL2)

Marty Hudzik November 5th, 2004 06:34 AM

In addition to that you can get better picture quality too. IF you set the highest DVD compliant data rate of around 9mbits/s you will get a 20% better picture (theoretically) as you are now only encoding 24 images a second instead of 30. Think of it like this.

9000000/30=300000 or about 300K per frame.

9000000/24=375000 or about 375k per frame.

ITs not a lot but it can give you better results.
Besides in my experience when I encode an mpg file that is 24P and I don't remove the 3:2 pulldown or it wasn't 24P 2:3:3:2 then the mpg encoder isn't efficient and the fiels based frames that are inherent in 3:2 cadences look like caca.

J. Lamar King November 5th, 2004 07:33 AM

It's still good to know that ISO number for your camera settings because it's easier to set lights while on set metering for footcandles. Then of course you can find the exact exposure with zebras or a scope.

Chris Hurd November 5th, 2004 09:16 AM

Well, regardless David, please do go ahead and post your results anyway.

David Lach November 5th, 2004 10:30 AM

Yes you're right Marty, looking at it this way, it appears the 24fps will be of a better visual quality regardless of the time of your project, since the 9mb/sec. limit is less distributed.

Charles Papert November 5th, 2004 10:32 AM

David, my experience has been that outside of prelighting a set (where it's more feasible to use a meter than set up the camera) or building up a key light, my meters tend to stay on the cart when shooting video. Especially when it comes to measuring contrast. The reason being that when in low light situations, levels that barely register on the meter can potentially register perfectly well on screen, but as you get to the top end of the latitude, the overexposure is so limited that I don't even trust a rule of thumb like "two stops over is completely blown out" because sometimes it is and sometimes it isn't. And then there's adjustable knee settings which can shift this aspect without affecting the rest of the curve significantly, not to mention gamma which shifts the midtone response, which can affect the results of a key/fill ratio.

In other words...a good monitor will tell you a lot more accurately what's going on than a meter. But it is nice to know what you are looking at for a median ISO, so you can get a rough idea of the lighting package for a given job (like in a Mini35 setup, where exposure is not a casual thing due to the light loss!)

David Lach November 5th, 2004 11:12 AM

Charles, how do you keep concistency throughout your different setups if not using a light meter? I understand the waveform monitor is more powerful and accurate than the best spot meter in the world, since in a way it gives you reflected light in IRE for every pixel in the frame, but how can you get a constant look over different setups without using your meter to determine key and ratios? Your eye is good enough to determine if it looks good for a particular setup, but might play tricks on you if you're trying to get your other setups to match the results from a previous setup.

Also, regarding settings, I concede the slight adjustment to pedestal, setup, gain, knee, etc. will change the way the cam behaves, and thus change the way it sees light and reacts to it, and just to make sure, I'll experiment with that too this week-end (trying to see how every setting affects a theoretical ISO value), but if you create one camera custom preset for your entire shoot, and get your ISO corresondance based on that preset, wouldn't the meter be accurate (enough) until you change those setup values again?

I'm a bit puzzled here because to me, it just makes sense to set the frame for the shot, use the light meter to get a very rough reading for your key, say more towards F2 or more towards F11 depending on the desired DOF, then fine tweak with the waveform monitor to avoid overexposure as well as zebra (and mine is set at 95 IRE, so that leaves a tiny bit of head room), use your meter again to determine ratios/zones, then getting back to the camera and waveform monitor to make final adjustments (kill hot spots, kill shadows bellow 7.5 IRE, etc.). This is the only way I can think of to simultaneuously get constant lighting setups throughout and expose well for the limitations of the video format (mainly less lattitude, a slightly different gamma curve, etc.).

Of course I understand that setting a 3:1 ratio between fill and key for example won't look and respond the same for film and video, but what I'm looking for is reproduceable and predictable results, adjusted around the camera's behavior under specific confitions with specific settings. Don't know if that makes sense or if there's still an element I fail to grasp.

Barry Green November 5th, 2004 01:05 PM

Quote:

Originally posted by Jeff Donald :
Video chips don't respond in a linear fashion.
I think Jeff is right. I did extensive testing with the DVX and found that the ISO response varied from 400 to 1280 depending on two primary factors: whether you were in interlaced mode, and how much light was in the overall scene. The DVX is about a stop faster in interlaced mode than progressive mode.

But in just progressive mode, at lower light levels (low enough that the camera's autoexposure recommended an f/2.8) the equivalent ISO rating was 400. Under much brighter conditions, where the camera was recommending f/16, the equivalent ISO response was 800. Depending on the gamma settings it could be as high as 1000.

Over most of the exposure curve the rating was 640, but could be a half-stop different at both ends of the spectrum. Definitely not a raw linear response.

(tested with a gray card and with real-life scenes, using a waveform monitor and two different spot meters).

Rainer Hoffmann November 6th, 2004 05:02 AM

Barry, of course no real world system responds in a perfectly linear fashion, especially at the ends of the spektrum. There will allways be a certain linearity error.

The difference between a CCD or CMOS chip and a film is, that the film "by design" does not respond in a linear fashion but in a logarithmic fashion while the chips respond more or less linearly.

A few days ago Jeff posted a link to some white papers on these topics on the Adobe website. Very recommended reading.

Jose di Cani November 6th, 2004 10:58 AM

amazing footage. It show what the xl2 can do. What a shame the thing is so expensive. I sure love that girl in the outside light. Very warm and friendly.

Mathieu Ghekiere November 6th, 2004 12:47 PM

The orange of the pumpkin is really beautifully rendered.

Dennis Hingsberg November 6th, 2004 03:28 PM

Thanks for all your feedback and comments. I'm about to start shooting a new short film titled "Little Acorns" and should have some stills (and possibly clips) to post in the next few weeks.

Also tomorrow, I'm on set for a 16mm short film being produced by a filmmaker friend of mine here in Toronto. I plan to bring the XL2 along and I'll see if I can't snag a few clips of some scenes - it would be great to see the results using a film lit scene.


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