Thread: When shooting RAW, is it better to overexpose or underexpose?

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  1. #21  
    Senior Member Tomas Stacewicz's Avatar
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    Kurt Lancaster in his 'Cinema Raw' states that the BMCC converts 16-bit linear to 12-bit log uncompressed RAW. So, it seems that the world between RAW and Logarithmic processing isn't as black and white as we concluded above. And we all know that the RAW image coming from the BMCC and BMPCC looks alot like the flat log profile. Compare with the URSA RAW which has lots of more colour and lacks the flat image of its predecessors.

    Any comments on Lancasters statement?
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  2. #22  
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    "Raw" as a term has come to mean many different things and now lacks specificity. In general, when people refer to "raw" or "camera raw" they're talking about the data stored in the specific raw container (DNG, R3D, ARI, etc). This data is usually a Bayer pattern (a form of CFA mosaic) and has not had any color transforms performed on it. Now, this does not mean that a transfer function (a.k.a. gamma curve) can't be applied to it. In the case of BMD cameras, they all perform a logarithmic transform on 16b linear data to save space in 12b DNGs. The purpose of this curve is to reduce the amount space taken up by the raw data and to retain details in the mids and shadows through compression.

    This data is still considered raw, and the general pipeline is as follows. This logarithmic raw data goes through decompression and linearization behind the scenes. Processing programs/pipelines prefer to work with linear data as it is easier to manipulate with simple operations. So the raw files contain the LUT to linearize the logarithmic data. After linearization, the Bayer pattern is demosaiced (a.k.a. debayering) to create an RGB image that still does not have any color transforms done. And many people confuse the debayering process as part of the color transform process. It has nothing to do with it. Its sole purpose is to create an RGB image from CFA mosaiced data. The color transform process is where you would choose the output/working color space for the image (BMD Film, Rec 709, ACES, etc). The color space is made up of the color gamut and transfer function. Usually, for convenience, the transfer function and color gamut are named the same as the color space, but is not always the case.

    So yes, raw data can be logarithmic or linear or encoded with any other curve you can think of. It's just a matter of choosing the right curve. In addition, raw does not have to be mosaiced. An example would be a camera with three sensors having a pixel for all three channels. The raw file wouldn't need to be demosaiced, but it would still need color transforms performed on it. Another example is actually the C500 2K raw. It super samples and creates 2K RGB raw from a 4K sensor.

    Now as for over or underexposing, it depends on where you want your dynamic range to lie. In the case of the original Cinema Camera and the Pocket, you'll get the following.



    What the chart shows is how the dynamic range is shifted with exposure. When you expose for 800, the BM recommended ISO, you get +5/-8 EV distribution. Now overexposing would be moving to the left. Exposing for 400 is the same as overexposing 800 by one stop. As you can see the range shifts down. This means you get cleaner shadows, but you will clip the highlights faster. In the opposite direction, exposing for 1600 is the same as underexposing 800 by one stop. This will give you an extra stop of range in the highlights, but means you will descend into noise faster.

    Hopefully this helps some and sheds some light on the subject for you.

    Here's an example with D16 going from raw data to color image.

    Log bayer data:



    Linearized bayer data:



    Demosaiced linear image:



    Final color image sRGB:



    or Final color image Bolex Log:

    Last edited by iaremrsir; 02-04-2017 at 02:01 PM.
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  3. #23  
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    @iaremrsir

    Where did that ISO/exposure chart come from? Is it based on actual measured performance, or is it just an extrapolation of how you believe the cameras operate?

    I ask, because I don't believe it's correct.
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    Quote Originally Posted by jrd View Post
    @iaremrsir

    Where did that ISO/exposure chart come from? Is it based on actual measured performance, or is it just an extrapolation of how you believe the cameras operate?

    I ask, because I don't believe it's correct.
    I made it based on a combination of information from the BMD Film curve, DNG metadata, independent and tests performed by Corey Robson and Ryan E Walters.

    I posted these a while ago:

    Quote Originally Posted by iaremrsir View Post
    Just thought I'd give an update to my graph with the characteristic of the BMD Film 4.6K curve! You can see that the new curve allows about a half stop more range in the highlights and about 1.5 stops more in the shadows than the original BMD Film curve.

    NOTE: To reiterate previous posts, this is data extracted directly from the LUTs provided by Blackmagic Design in Davinci Resolve.



    Here is the data for all of the curves on a 10b scale:

    BMD Film:
    1% Black: 36
    18% Gray: 392

    BMD Film 4K:
    1% Black: 36
    18% Gray: 392

    BMD Film 4.6K:
    1% Black: 76
    18% Gray: 420

    And the linear (horizontal) signal vs the BMD Film signals:
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  5. #25  
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    Quote Originally Posted by jrd View Post
    Yes, and Santa Claus is also coming to town. There are some advantages to raw under some circumstances, as well as downsides, in other circumstances -- the way raw allocates bits isn't necessarily the most effective or efficient for every scene. And in low-light situations, you may even be better off shooting in rec. 709 video mode than either Prores film or raw.

    There are no miracle formats.
    I'd like to see this proven, because right now I'm pretty skeptical. Especially for RAW. For Film, it makes sense in theory, but I have doubts it would make a practical difference.

    Quote Originally Posted by jrd View Post
    @iaremrsir

    Where did that ISO/exposure chart come from? Is it based on actual measured performance, or is it just an extrapolation of how you believe the cameras operate?

    I ask, because I don't believe it's correct.
    The numbers on it are subjective, and you lose dynamic range on lower ISO's (Edit: in ProRes) but I believe the concept that it's demonstrating is the point iaremrsir is making.

    To answer the question in the OP, provided you're not clipping important data, it's better to overexpose RAW because it gives you a better signal to noise ratio. Obviously, everyone has different tolerances when it comes to noise, and sometimes noise is desirable, but assuming a normal use case, overexposure is preferred. I typically just shoot at 400 ISO/ASA, which overexposes a stop. If you have time for noise reduction or want a little grittier image, 800 ISO has the most dynamic range and is proper exposure for the camera. 1600 is underexposed by a stop and is too noisy for me.

    EDIT: Whoops, sorry, didn't have my Wheaties today.
    Last edited by RyLo; 01-04-2017 at 06:09 PM. Reason: Math is hard.
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  6. #26  
    Senior Member Tomas Stacewicz's Avatar
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    Thank you iaremrsir and RyLo for your answers! Really made it much clearer for me seeing those diagrams and curves, and the comments. :-)

    Is it ok to quote you both on my blog?
    Last edited by Tomas Stacewicz; 01-04-2017 at 07:00 PM.
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  7. #27  
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    Quote Originally Posted by RyLo View Post
    The numbers on it are subjective, and you lose dynamic range on lower ISO's (Edit: in ProRes) but I believe the concept that it's demonstrating is the point iaremrsir is making.
    These actually aren't subjective numbers. These are, as stated before, based on the data from the BMD Film curve LUTs, DNG metadata, and independent tests. The independent tests were just used as supporting evidence. I provided the math behind calculating these numbers for generating the charts on the Blackmagic forum a while back. I wanted to be as objective as possible in making these charts. A lot of the time, there is still valid data in the noise floor that can't be viewed on a scope, but is still visible in the image. The data is there at the bottom and you'd be able to see it in a log image, whether it's considered usable or not is up to the user.

    Quote Originally Posted by Tomas Stacewicz View Post
    Is it ok to quote you both on my blog?
    I'm cool with it.
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  8. #28  
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    Quote Originally Posted by iaremrsir View Post
    Exposing for 400 is the same as overexposing 800 by one stop. As you can see the range shifts down. This means you get cleaner shadows, but you will clip the highlights faster.
    All I'd suggest is, get hold of a BMCC or BMPCC and actually test that claim (for Prores Film mode). Where the values are mapped on a waveform does change, but I believe you'll find that you won't clip highlights any sooner at 200 than 800 (you'll be at exactly the same F-stop). Which (if true) raises questions about your chart?
    Last edited by jrd; 01-04-2017 at 09:12 PM.
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  9. #29  
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    Quote Originally Posted by jrd View Post
    All I'd suggest is, get hold of a BMCC or BMPCC and actually test that claim (for Prores Film mode). Where the values are mapped on a waveform does change, but I believe you'll find that you won't clip highlights any sooner at 200 than 800 (you'll be at exactly the same F-stop). Which (if true) raises questions about your chart?
    I see where the misunderstanding is now. It sounds like you think I'm saying that simply switching the ISO value in camera will cause the shift in the dynamic range. You're right in saying that without changing the aperture or shutter speed or any other physical factor, that all that will happen is changing the placement of the mapping. But what I said was "exposing for [ISO]" which involves changing the physical amount of incoming light so that middle gray is recorded at the correct value in camera.
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  10. #30  
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    Eddie, thank you very much for your clear and detailed post, your contribute to this forum is really precious!

    PS. any news about a new CCD cam? I remember an old discussion on BMCuser
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