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Exposing correctly for video


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#1 Luke McMillian

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Posted 16 February 2007 - 10:13 PM

I am just wondering what is the best way for exposing video. I shoot on a prosumer mini dv, I usually set up the lights and use zebras to tone down my fstop a bit so there is very little zebras showing and nothing is blown out, and I have exposure lock that I usually lock down on that and shoot.

But I've heard of some people just setting up their key light only and zooming in on the person and exposing just their face with the key light, lock the exposure and then zoom back out and switch on their other lights, fill and backlight etc they had previously set up, Then tape their wide shot. Never heard of just exposing for the key light...

Or I've heard of some people setting up all their lights and putting an 18% gray card in the frame under the lighting and expose for that by locking the exposure down, then zooming back out and recomposing their shot. i know all cameras are different but what is the usual method for correct exposure and how is it done on the high end video cameras versus a prosumer mini dv?

thanks all!

Luke
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#2 Jonathan Bowerbank

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Posted 17 February 2007 - 12:19 AM

In video, generally you want to underexpose by a stop or even possibly two. When you overexpose or even get "correct" exposure, you lose a lot of information in highlights, and in video that's detail that you can't get back by correcting in post.

So essentially, underexposing gives you more information to work with and more detail to bring up in post.

What I usually do is set up a grey card, expose for that, set my light meter to be relevant with the camera's reading, then underexpose my key by a stop.
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#3 David Mullen ASC

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Posted 17 February 2007 - 12:50 AM

Because I don't like noise, I'm not so keen on the "underexpose everything" approach to video. But often I do just that, but it really depends on the highlights in the shot.

In other words, I base the exposure on the faces, usually setting the zebras to appear at 70 IRE and then underexposing caucasian faces just below that zebra point (so maybe only the shiny bits of the face zebra.)

However, I look at the bright areas in the frame and if there is important detail in danger of being clipped, I bring down the overall exposure just enough to hold that detail. But otherwise, I'd prefer not to underexpose too much. I've had just as many problems color-correcting underexposed video in post as I do video with some clipping (which can't be fixed anyway.)

Exposing video is really about recognizing you have a set range to place exposure detail in, basically 0 IRE to 100 IRE depending on the camera, format, etc. It's OK to let tiny areas in the frame get clipped -- if the sun momentarily glints off of a car or someone points a flashlight into the lens, etc. those areas can burn out. What you don't want is some large and distracting area of the frame being clipped, like a shirt someone is wearing, or a sky with clouds in it.
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#4 Luke McMillian

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Posted 17 February 2007 - 12:56 AM

Hey,

For someone like myself that doesn't have a light meter and is just on a simple set up, how would you do something similar to what you said with your set up. I don't understand the whole grey card thing, do you zoom in on the grey card and fill the frame with it? I'm realying on the zebras right now, so I don't the exact correct procedure for how to do it. Should I be just figuring out my f stop from just my keylight, and then unde4r expose so I'm not blowing out the highlights, and then shooting everything else in the scene and Close ups and cut aways at the same f stop? What is the prefered method of working. I can't seem to find this information in any book! What is your set up going from lighting to recording with the camera, and through out the rest of the scene, just technically with regards to lighting and exposure properly for best results.

Thanks

Luke
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#5 David Mullen ASC

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Posted 17 February 2007 - 01:15 AM

Hey,

For someone like myself that doesn't have a light meter and is just on a simple set up, how would you do something similar to what you said with your set up. I don't understand the whole grey card thing, do you zoom in on the grey card and fill the frame with it? I'm realying on the zebras right now, so I don't the exact correct procedure for how to do it. Should I be just figuring out my f stop from just my keylight, and then unde4r expose so I'm not blowing out the highlights, and then shooting everything else in the scene and Close ups and cut aways at the same f stop? What is the prefered method of working. I can't seem to find this information in any book! What is your set up going from lighting to recording with the camera, and through out the rest of the scene, just technically with regards to lighting and exposure properly for best results.
Thanks
Luke


Your subject's face in the key light is one of those "highlights" -- what you really mean is "bright highlights", the ones brighter than the key.

Obviously one method that would help is to light each shot in a scene to the same f-stop.
You could do that with a light meter. Or the way in which I do this in HD is to set the lens to the f-stop I want to use (like f/2.8 let's say) and then light & adjust the lighting levels until it looks correctly-exposed on the monitor and viewfinder. After that, if you don't touch the f-stop and the monitor set-up, you can tell if the next lighting set-up is too bright or not just by looking at the monitor.

Face looks too hot at f/2.8? Drop a scrim in the light, move the light back, etc. Window too hot and clipped at f/2.8? Put some ND gel on it, put a scrim on a frame outside the window, etc. Sunlight streak on the floor too hot at f/2.8? Use a net flag to darken the light on that area, or use an ND grad filter on the camera for that area.

Look, usually something in the frame is going to "blow out" (clip) unless you have a really boring, low-contrast scene with no hot highlights. You just don't want excessive amounts of clipping. I mean, on the good monitor, you should be able to see these clipped highlights anyway.
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#6 Robert Aldrich

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Posted 17 February 2007 - 01:28 AM

Hey,

For someone like myself that doesn't have a light meter and is just on a simple set up, how would you do something similar to what you said with your set up. I don't understand the whole grey card thing, do you zoom in on the grey card and fill the frame with it? I'm realying on the zebras right now, so I don't the exact correct procedure for how to do it. Should I be just figuring out my f stop from just my keylight, and then unde4r expose so I'm not blowing out the highlights, and then shooting everything else in the scene and Close ups and cut aways at the same f stop? What is the prefered method of working. I can't seem to find this information in any book! What is your set up going from lighting to recording with the camera, and through out the rest of the scene, just technically with regards to lighting and exposure properly for best results.

Thanks

Luke


I've had the same question in the past and it was only partially answered for me, but I realized just now, that the purpose of the gray card is to get a starting exposure for your shot, and then adjust for exposure on things in the shot that matter. (Of course, this is pretty much exactly what was just said above)

People and things aren't 18% gray, after all.

So tweak the image from that exposure, so people's faces/skin look good, and there aren't any blown-out areas that look bad. It's a trade-off sometimes, but you can't always control all the places the light is hitting and adjust that, so to save time/money you can adjust the exposure if you feel it doesn't degrade the image too much.

And there is occasionally that time where you set your auto exposure and the image looks exactly the same as it does when you set everything manually...and everything looks gorgeous! Makes you proud!
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#7 Luke McMillian

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Posted 17 February 2007 - 01:55 AM

So informative Mr.Mullen thank you very much! You sound like a true artist the way you work, creative problem solving! That's the level I want to get to eventually. Does each DOP have their own unique way of working, or is the usual way :

seeing the blocking and hear the artistic requirements of a scene from the Director,
then setting up your lights
then pick your f stop reading from the lighting using a light meter, or the metering on a video camcorder
base all the other shots in the scene around that f stop and adjust lightings, nd gels, scrims etc like you said

Or do you always decide creatively on a f stop for the entire scene, and do the method you previously described? Because most scenes in a movie use a low f stop for shallow depth of field shots for close ups/over the shoulders, and I believe most master shots are shot with a narrow f stop to give a greater depth of field, correct me if I'm mistaken. Is then the advantage to working with an f stop through out a scene is giving you more or less similar exposure from shot to shot?

Any resources/books/web sites you'd recomend for me to find this type of information? I want to get in the habit of learning the whole correct shot set up technically with my mini dv gear so I'm ready to tackle on bigger jobs in HD. Out of all the books I've read, I've never come across this basic information.

Luke
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#8 Jonathan Bowerbank

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Posted 17 February 2007 - 02:39 AM

Look, usually something in the frame is going to "blow out" (clip) unless you have a really boring, low-contrast scene with no hot highlights. You just don't want excessive amounts of clipping. I mean, on the good monitor, you should be able to see these clipped highlights anyway.


I always need to hold in a chuckle when I'm watching a noob DP work, and suddenly he's freaking out about a reflective reading on his meter that shows an area of the frame being blown out. A well placed highlight is always nice, just when I'm speaking in matters of what my key is, I tend to underexpose that area by a stop or so when shooting video.

:)
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#9 David Mullen ASC

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Posted 17 February 2007 - 03:04 AM

Generally you light all the angles of a scene to the same f-stop, partially because your using the same lights and you're not completely relighting each time, just adjusting for close-ups, etc. So it's easy to light each shot to the same f-stop.

Now in video sometimes, I find it hard to light to a low f-stop, especially when trying to balance with a bright window or something. So I sometimes light to a higher f-stop and use an ND filter in order to shoot at a wider aperture. So in that case, I may go ahead and shoot the master at a deeper stop where I don't need shallow focus and would rather have the lens be sharper, then switch in the ND and shoot the close-up wide-open on the lens - but even in this case, all the shots are lit to the same level, I'm only messing with the ND filter on the camera. An example would be a day interior lit with HMI's to an f/5.6 -- I could switch in the 2-stop ND filter to shoot at f/2.8.

If you light your master shot well-enough, then you shouldn't be relighting too much for close-ups. Starting from scratch on each set-up can lead to mismatching.
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#10 Dan Salzmann

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Posted 17 February 2007 - 09:30 AM

"If you light your master shot well-enough, then you shouldn't be relighting too much for close-ups. Starting from scratch on each set-up can lead to mismatching."
Very true and very important. It can also lead to scheduling problems when things are taking too long.
All that was said about balancing interior light to in shot windows is also true but in "Inland Empire" the windows are often way blown out and it didn't bother me.
I don't like video noise however. Increased gain just doesn't look good to me.
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#11 Chris Durham

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Posted 17 February 2007 - 01:24 PM

I base the exposure on the faces, usually setting the zebras to appear at 70 IRE and then underexposing caucasian faces just below that zebra point (so maybe only the shiny bits of the face zebra.)


I know I'll experiment myself, but using this methodology as a jump-off point, how should I compensate for the fact that my camera's zebra only goes down to 80 IRE? ND Filter? Underxpose by a certain stop?
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#12 Michael Nash

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Posted 17 February 2007 - 05:41 PM

I know I'll experiment myself, but using this methodology as a jump-off point, how should I compensate for the fact that my camera's zebra only goes down to 80 IRE? ND Filter? Underxpose by a certain stop?


You could try underexposing by about 1/2 stop, depending on the gamma setup you're using.

It's true that a "normal" exposure of Caucasian skin will place those tones mostly between 50-70 IRE. But really, the best way to "correctly" expose video is to use a properly setup monitor in controlled viewing conditions (a shaded screen away from bright light). When it looks right on the screen, it's right! In drama for instance, that may place your skin tones all over the place. At that point zebras are only a reference of what's happening in the frame, not a suggestion for exposure. No different than a spot meter.

It's also true that if highlights clip, they're gone forever. And that you can recover underxposed material better than overexposed. But it's most common and usually best to try to "nail" the exposure in camera, underexposing for highlights when necessary.

That's not to say that consistent and deliberate underexposure, with the intent of recovery in post, is "wrong." It's really not that different in concept from underrating a film stock and "printing down." But as a professional video shooter, I can pretty much count on the fact that when I hand over a tape to the producers, that footage is going to be used as is, with a minimum of correction on the master. That gets you used to getting it "right" in camera.
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#13 Frank Barrera

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Posted 17 February 2007 - 08:16 PM

I just did a little job with the DVX100. The job was so little that they did not provide me with a proper field monitor. So there I was shooting an interview of a woman with fairly reflective skin tone and working with the 80IRE set up. It was a bit disconcerting to expose based upon that little flip out screen. I found myself popping on the color bars from time to time just to remind me what the proper angle of view is to evaluate brightness and contrast. I would say I was a good 2 stops below 80IRE on her face for good exposure.

When shooting with a proper video camera I set the zebras to 70 and 100. This makes sense.

Why in the world would any video camera stop at 80IRE?
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#14 Chris Durham

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Posted 17 February 2007 - 08:38 PM

I don't know; but the XL2 lets you set to 80, 85, 90, 95 or 100. That's it.
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#15 Michael Nash

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Posted 17 February 2007 - 10:17 PM

I would say I was a good 2 stops below 80IRE on her face for good exposure.

When shooting with a proper video camera I set the zebras to 70 and 100. This makes sense.

Why in the world would any video camera stop at 80IRE?


I don't doubt that you got good exposure wherever you ended up, but two stops sounds like too much to bring 80 IRE down to 70 IRE. It's more likely that you were exposing her face much darker than "peaking" at 70. I'm not suggesting you exposed incorrectly, I just want to point out that two stops does not equal the difference between 70 - 80 IRE -- in case anyone tries to use that as a rule of thumb.

On a broadcast camera with a relatively "normal" looking contrast, you'll drop between 10-20 IRE units per f-stop starting at 50 and underexposing, becoming less as you near the "toe" of the image. 15 IRE per stop is common, although I usually run closer to 10 for a more natural-looking shadow response.

On the overxposure side though, it's a much steeper curve. An increase of one f-stop above 50 gets you up to 65, another pushes you past 80. The Cine-like gamma of the DVX would flatten that out some, but I still wouldn't expect the difference in 70 - 80 IRE to exceed one stop.

Of course you really need to map this out with a waveform monitor, and I haven't tested the DVX this way yet. I may soon though; a company I work for recently purchased a 100B, and I want to try to match it to the Sony D35 they also own. I'll try to post results when it happens.
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