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Old November 6th, 2013, 02:27 PM   #1
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Exposure - What is "correct" - Science or opinion

I really enjoyed the post on white balance in this forum, and thought I would open a discussion about exposure, hoping some of the very knowledgeable people on this forum share their thoughts.

While most of us understand the basics of exposure- "Thou shall not clip," using histograms and their limitations, waveforms, and the zebra debate -70% -works or not, Id love to learn more about what is "proper" exposure.

In a documentary setting, where shooting is fluid and often outdoors, a cinematographer is always on their toes. I generally use 70% zebras just on the extreme highlights on a face, as I mainly am filming people. I guess where I feel lost sometimes is landscapes. If I make sure that nothing that I deem important is above 100%, is it up to my eye to judge what is "correct" beyond that?

I feel like my images are correctly exposed, but am wondering others techniques for judging exposure. I look forward to learning.
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Old November 6th, 2013, 02:39 PM   #2
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Re: Exposure - What is "correct" - Science or opinion

A proper conversation on exposure must be done in context with the gamma curve of the recording instrument.
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Old November 6th, 2013, 02:59 PM   #3
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Re: Exposure - What is "correct" - Science or opinion

Quote:
Originally Posted by Annen James View Post
I generally use 70% zebras just on the extreme highlights on a face, as I mainly am filming people.
The danger with a statement like this of course is the variables.

My USUAL statement of my preferred practice, all other things being equal, is:

"I expose for 75IRE zebra on the highlights on normally reflective caucasian skin tones under NORMAL lighting where I'm not trying to create a 'look'".

On particularly dark skin tones, one also needs to consider reflectivity. I shot a bunch of promo stuff for a semi-pro basketball league once and while the preponderance of players were of a very dark skin tone, because they were SO reflective due to sweat and overall reflective skin, I exposed for zebras on highlights on dark skin tone at 85 or 90 IRE and was pretty much bang-on for exposure.

When we shot in India in '05, I used my own skin to set exposure because I am a known commodity to me. Every shot turned out perfectly exposed, regardless of skin tone on our interview subjects.
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Old November 6th, 2013, 03:35 PM   #4
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Re: Exposure - What is "correct" - Science or opinion

What is correct, is knowing the science so that you can use it to obtain exposure that is in your opinion correct for what you are trying to accomplish.
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Old November 6th, 2013, 05:42 PM   #5
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Re: Exposure - What is "correct" - Science or opinion

I think in many ways exposure for digital imaging is actually really nice and simple. Because cameras read luminance in simple IRE values from 1-100 (well, 1-109 in some circumstances but let's not complicate things) so we can very easily transpose the Zone System to digital.

All you need to know is where 'white' and 'middle grey' fall within the specific gamma curve you're shooting with, then you simply expose to place your elements within the frame at the specific level of brightness you want them to be (all of which is made dead simple, and really easy through the use of false colour and waveform monitoring tools).

That's really all there is to it - know where white and middle grey should be and expose accordingly.

Now obviously there are going to be times when the contrast ratio in a scene means you'll have to make compromises to either your shadow or highlight detail, and there will also be times where for the sake of avoiding noise you might boost your exposure in a low-key shot (with the plan to bring it back down in post). But so long as you know where white and middle grey need to fall, you'll know exactly how much you're pushing things when you have to push them.
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Old November 8th, 2013, 10:58 AM   #6
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Re: Exposure - What is "correct" - Science or opinion

If I understand the OP correctly I don't think he was asking us to go directly to the gamma curve specific to his instrument.

I believe he is asking for tips and techniques we use to calculate exposure possibly using everything from just the tools a camera offers in EFP situations to wave form monitoring. There are many ways to do it and after more than twenty years at it I am always looking for new information myself.

Annen,

I very long time ago I was trained with Betacams. Zebras were about the only scientific tool those old cameras offered. I was taught to run it at 80% and hope I could set exposure to include a small amount of zebras on the highlights of the subjects face, regardless of skin color. That was the starting point. Then it was up to the operator to use his knowledge, experience, and good judgment to dial it in. It was far less technical then. Especially when working in the field run & gun. And all you had was a black & white viewfinder and no LCD screen. But somehow we got it right without knowing the gamma curve. We had too, or you got fired immediately.

My latest camera purchase is a Sony EA50, an entry level pro camera. On the zebra settings in the menu it has a wide range going from 70 to 100+. So even after all my years I have to ask, what the hell is 100+? So far I am assuming they are saying that at 100 you are technically not clipped yet, but if you set for 100+ and you see stripes you are definitely clipped. Can someone please clear that up for me? Even if that is what it is supposed to mean I would never rely only on that. For one thing, zebra stripes are not that accurate. My technique is to fall back on experience and my mind and eye are always instinctively assessing the scene for white, gray, black, and extremes in brightness. So for me, when wave form monitors are not available to paint with and study, exposure is a combination of rudimentary science like zebras, and histogram, combined with judgment and experience. My judgment will trump zebras every time.

On the main home page of DVINFO.net there is a recent article on wave form monitoring by Art Adams. It is not how to read a monitor 101 but it is an excellent article.

I will try to contribute more later, but, like you, I am very interested in reading the other guys tips. They are often more informative than mine. I am always willing to learn.

Steve
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Old November 8th, 2013, 11:09 AM   #7
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Re: Exposure - What is "correct" - Science or opinion

The issue with ignoring the gamma curve of the instrument and discussing just the tools is that some of the tools will not work if you use one of the more aggressive gamma curves such as s-log. Putting skin tones in the 30-50% IRE range as you would with s-log won't allow you to use zebras since they don't go down that low on any of the cameras I have.

The real answer to the question is to know how the instrument you are using records light and put the object of interest in a portion of its response curve that is not overly compressed. That allows you to hold as much detail as possible on that object and create the best rendering of it. You have to then consider where the rest of the image falls along the luminance range and evaluate if you are loosing too much data to create a balanced and pleasing overall image.

This is an issue to complex to tackle without referencing it to a specific response curve.
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Old November 8th, 2013, 01:47 PM   #8
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Re: Exposure - What is "correct" - Science or opinion

Quote:
Originally Posted by Steven Digges View Post
And all you had was a black & white viewfinder and no LCD screen.
Yes but the B&W viewfinder was a high contrast CRT that didn't have the issues that small LCDs do in terms of panel gamma.

I miss having a full raster B&W CRT for exposure and focus.
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Old November 8th, 2013, 01:55 PM   #9
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Re: Exposure - What is "correct" - Science or opinion

I agree Shaun. And they worked in all lighting conditions.
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Old November 8th, 2013, 07:48 PM   #10
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Re: Exposure - What is "correct" - Science or opinion

I use an ikan d7w field monitor that does false colors. I find false colors to be a powerful tool for setting up a multicam shoot, or managing the exposure of a single camera shooting in variable conditions over time. With a simple glance the operator can analyze the exposure the whole scene. If the scene is too dynamic, he can make a decent compromise.
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