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Understanding IRE


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#1 Chris Durham

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Posted 14 June 2007 - 05:04 PM

I've recently noticed that footage captured on my XL2, which looks fine in the viewfinder, comes out pretty dark when dumped to computer. One reason I've considered for this is that some time back I set my Zebra settings down to 90 IRE, and since I'm using Zebra as my reference (can't afford a Waveform Monitor right now) that would lead me to stop down or apply an ND before I would have to if my Zebra was set higher. I'm going to experiment with the setting some to see if I get better results when I reference Zebra set at 95 or 100 IRE.

But that led me to wonder, being pretty new to all this, how well I understand IRE, particularly as it applies to a color image. For instance I know that 100 IRE represents white; but is that white or is it "white?" If I'm aiming my camera at an orange flame, for example, and I get zebra in the flame, does that mean that one color, probably red, is overexposed? Am I losing less detail than I would on, say, a cloudy sky where all 3 colors are clipping? With my flame, since green is involved as well (and blue to a lesser degree), am I retaining some detail from that channel?

I guess what I'm asking is whether when we talk about exposure in terms of IRE, are we talking about exposure per channel of color? And if I see Zebra in my viewfinder is it likely then that it might only be one channel overexposed (unless, of course, I'm looking at something white?).
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#2 Jonathan Bowerbank

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Posted 14 June 2007 - 06:03 PM

I usually don't rely on Zebra for setting my exposure. It's good to know if something that white and blown out is appearing in my frame, and it does give me a hint that I should stop down if TOO MUCH of the frame has zebras.

Generally, I set my auto-exposure to a grey card at the beginning of production and set my light meter to the ASA that corresponds to that f-stop. Then work manually from there, not worrying too much about what's zebra'd.

If something has gone way over IRE and isn't broadcast safe, there are settings in your NLE to bring just THOSE WHITES down to make your footage broadcast safe.
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#3 Daniel Sheehy

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Posted 14 June 2007 - 09:39 PM

The IRE reading is the readout of the voltage amplitude in the video signal. In-camera, it is most likely reading the luma signal, which will give you no indication of which colour channel is doing what.

Theoretically, you could split it into the full RGB channels & run each one through a WF monitor...

If your footage is looking dark on your computer, but ok in the VF, I'd be checking your VF display settings first, and also you computer display.
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#4 Bruce Greene

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Posted 14 June 2007 - 11:23 PM

I've recently noticed that footage captured on my XL2, which looks fine in the viewfinder, comes out pretty dark when dumped to computer. One reason I've considered for this is that some time back I set my Zebra settings down to 90 IRE, and since I'm using Zebra as my reference (can't afford a Waveform Monitor right now) that would lead me to stop down or apply an ND before I would have to if my Zebra was set higher. I'm going to experiment with the setting some to see if I get better results when I reference Zebra set at 95 or 100 IRE.

But that led me to wonder, being pretty new to all this, how well I understand IRE, particularly as it applies to a color image. For instance I know that 100 IRE represents white; but is that white or is it "white?" If I'm aiming my camera at an orange flame, for example, and I get zebra in the flame, does that mean that one color, probably red, is overexposed? Am I losing less detail than I would on, say, a cloudy sky where all 3 colors are clipping? With my flame, since green is involved as well (and blue to a lesser degree), am I retaining some detail from that channel?

I guess what I'm asking is whether when we talk about exposure in terms of IRE, are we talking about exposure per channel of color? And if I see Zebra in my viewfinder is it likely then that it might only be one channel overexposed (unless, of course, I'm looking at something white?).


Chris,

You asked some really good questions here. I hope I can help a little.

#1 Your camera probably records up to 109% rather than just 100%
#2 If you set the zebra to 100%, it's still possible that one, maybe even two, colors are clipping when the zebras appear.
#3 Judging your exposure on your computer monitor might be misleading. On a Mac for instance, the Quicktime codec for DV assumes that you have your monitor set to a gamma of 1.8 (a mac tradition). If you've set your gamma to 2.2 (the pc tradition, and the gamma correction for a typical CRT tv) then your play back will look a little dark on a Mac. Of course, most photographs on the web are set to a gamma of 2.2 and such will look too light on a Mac set to a gamma of 1.8. On a Mac you can change the gamma by checking the settings in the monitor display preferences. You should also have your monitor calibrated using an instrument.

Since you are shooting mini dv, the best way to judge is to connect the firewire cable to the camera and connect the camera to a broadcast monitor, or at least a tv.

#5. When I'm shooting, I like most to set my exposure by consulting a professional monitor and a waveform set to display RGB separately. When I'm on the run, and that is impossible, I set my zebra to 100% knowing that I have a little headroom above 100% before white out clipping (109%) The challenge with the XL2 is that the little viewfinder is a poor way to judge the brightness of the mid-tones. Try to use a real crt monitor to judge the overall exposure. Keep in mind that there will be times when you'll want to have some clipping in the image in order to expose the overall image to the best effect. I'm thinking of glints off of cars, practical lights in the frame, bright exterior windows, sometimes backlight on people. Also keep in mind that clipping a white curtain or wall does not look nearly as "electronic" as clipping a colored wall or worse, a face.

Hope this helps a little....
-bruce
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#5 Walter Graff

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Posted 17 June 2007 - 10:52 PM

I'll lay odds your problem is in your computer monitor. How does it look on a real monitor? If it looks good, then the gamma in your computer set up is incorrect, and/or your overall monitor setting on your computer is off. Also turn on your waveform in your edit system and see how high it reads. Does it read exposure the way you recorded it? If so, then your monitor is your culprit. Zebras measure luminance so if it's over the limit it's over. Set your zebras at 100% and you'll see them ring at 100%. individual color has little to do with it. This is a problem for most folks who don't realize that when dealing with video and computers monitors you are dealing with different standards so unless you first calibrate all your monitors (camera, computer, and external), you may never know if you've got accurate color or exposure.
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#6 Alessandro Machi

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Posted 18 June 2007 - 01:20 AM

I would bet you are doing zero IRE for base black, and therein lies your problem.

I can re-correct a scene at either 70 IRE or 100 IRE on the top end of the scale, but if I'm off just one or two IRE on the bottom end of the scale, I've done way more damage. Stay at 7.5 for your base line black. Also, when you shoot, REDUCE your contrast, don't extend it. don't use 0 to 109 IRE, use 7.5 to 109 and you'll be much happier. If you can reduce your top end, do 7.5 to 105.

I absolutely believe in using the zebra's, that's the primary way to prevent those annoying hot spots on the end of noses or way too blown out hairlights. However, zebra's will just hamper you if you are doing zero IRE on the base line. You have to raise your baseline black to 7.5 otherwise you are just recording way too much contrast that the tape cannot handle.
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#7 Andrew Nicholson

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Posted 21 October 2007 - 09:48 AM

It's not going to be everyone's ultimate solution but if you have a lappy get using "Serious Magic's DV Rack" for calibration, framing, focus, DV capture and pretty much every other thing relating to the camera on a shoot. This program includes wave form monitors and the works, it's everything you need in one little lappy...
If Barry Green recommends it, I'd say you have a winner!
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#8 Jack Brandhorst

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Posted 22 October 2007 - 01:07 AM

I usually don't rely on Zebra for setting my exposure. It's good to know if something that white and blown out is appearing in my frame, and it does give me a hint that I should stop down if TOO MUCH of the frame has zebras.

Generally, I set my auto-exposure to a grey card at the beginning of production and

set my light meter to the ASA that corresponds to that f-stop.

Then work manually from there, not worrying too much about what's zebra'd.

If something has gone way over IRE and isn't broadcast safe, there are settings in your NLE to bring just THOSE WHITES down to make your footage broadcast safe.


Hey Jonathan,

Is there any way I could get you to elaborate on how to expose using a light meter. Forgive my ignorance but i don't know how ASA and fstop correspond w one another.
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#9 Adrian Sierkowski

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Posted 22 October 2007 - 07:07 AM

Essentially ASA is the sensitivity of the recording medium, normally film, and the F-stop is the size of the iris aperture which lets through light.
So, what you'd do, is expose the gray card the way you'd like it. Say your camera is now at F11, you go to your light meter, and you meter the card, a spot meter would be best, and you adjust the ASA setting on the meter until it too corresponds to F11.
For example, you may find that for your particular camera, say an HVX, that a gray card is properly exposed at F11 and your light meter reads F11 when you set it's ASA to 320. Therefore the effective ASA for the HVX is around 320 and you can use the meter now to gets your exposures pretty spot on.
For the XL2, I believe he ASA is somewhere between 400 and 500, but it's been awhile since I've used one so I'm not 100% sure.


Hope that helped!
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#10 Walter Graff

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Posted 22 October 2007 - 07:34 AM

Of course since the s curve of a video camera doesn't follow as uniformly as film on its toe, your EI (called ASA here) may be considerably different depending on the light levels. For instance a camera that is measured using these rough ways of measuring at 320 in indoor 2000 lux of light may be 640 in outdoor light, so while this method can ball park you in a particular lighting situation, it's got a lot of holes. Blame that on video chips, the design of such things as microlenses and the nature of how an electronic CCD sees.


While in terms of definitions, a filmmaker using a video camera may wish to use film tools, there really is no such thing as a light meter for video (in the same sense as film). I would not use a waveform scope to expose film just as I would not want to rely on a film style tool to accurately tell me about video. The single two best methods of measuring levels and exposure with video in order of importance are 1, a waveform/vector scope which gives you all parameters of exposure level and color with 100% accuracy and 2, a properly adjusted television monitor which shows you exactly what the camera is seeing.

Outside of that, other tools such as light meters are great for general purpose when calibrated roughly for a particular set up but learning to use a light meter with video so that it is accurate over time is time consuming and really not a great way of measuring video levels even if we use the nomenclature of shooting a film. It's still video whether its got the blur of film or not and the best tools to use are always proper video tools. I would not use a bike pump to blow up a car tire and while it does work, I've only used it when the proper tools for a car were not available, and even then, it was less than perfect.

Please keep this in mind as there are good ways to do things and ways that while they seem to be correct, are simply not the best resources for accuracy, especially if you are not familiar with these tools for what they were designed for in the first place. If a light meter was made to be used with a video camera, whether it has a matte box, or not, then we'd have been using them since video was invented. We have, but only in controlled studio situations and even then only to understand the evenness of levels over a stage area and the like, not to accurately gauge actual exposure levels of a camera. That is what waveforms and monitors are for.

And actually the only 'accurate' way of gauging a light meter to use with video involves a chip chart, a light and a waveform. And even then its' only for a particular light level and iris setting. Using any other method is even less accurate.

You can tighten a screw with a knife from your kitchen but a screwdriver does it so much better and a knife is great at buttering toast.

Edited by WALTER GRAFF, 22 October 2007 - 07:38 AM.

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#11 Hal Smith

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Posted 22 October 2007 - 01:33 PM

In a controlled studio situation often it's the Lighting Director who has a lightmeter. Ultimately it's their job to deliver the proper level and ratios of illumination for the production at hand. An experienced LD can hit given levels and ratios by knowing his/her equipment, then they might confirm the design with a meter. In truth TV studio lighting is often somewhat boring since it has to work over large areas without any black holes, therefore the classic "flat" TV look.

I lit a stage production this year for a Director with TV experience. He wanted a very even, filled in look for the production. I hated what it looked like to the eye in performance but boy did it look good on an archival tape I shot and on a local station's news segment on the play's star. (The actress was an OKC native who has made a career for herself on Broadway therefore the local TV interest).
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#12 John Sprung

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Posted 22 October 2007 - 01:43 PM

Theoretically, you could split it into the full RGB channels & run each one through a WF monitor...

Not just theoretically. There are lots of WFM's used in post that give you four displays in a row: lumanace, red, green, and blue.



-- J.S.
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