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  1. #11  
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    It is useful to think of native ISO as defining optimum exposure sensitivity required to saturate the sensor with light in the same way you would expose film to get a rich, full dynamic range image. As a practical reference point you have typically +/- 2 stops of exposure index adjustment to optimize exposures for scene dynamic range for best results without losing much IQ.
    The physical sensitivity of silicon to light is a hard fixed value that varies a bit depending on how it is doped with other elements, but that value is pretty uniform across all digital cameras at 400-640 ISO for middle gray at 5000K color temp. The other thing that affects variations in camera sensitivity is how saturated the bayer color filters are. More saturated filters create richer images with more refined color discrimination at the cost of typically about a stop of sensitivity relative to lower saturation filters optimized for low light sensitivity.
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  2. #12  
    M harvey, you make an excellent point, but for different reasons than you might have thought. The process of encoding RAW to Log3G10 should be extremely high quality and it should not matter where one is on the exposure range, as long as one doesn't clip.


    However, there is noise and there is clipping, and that DOES affect how the compression processes do work in the real world, where one wants to allocate as much bandwidth to the "real" signal without overallocation bandwidth to parts of the signal that will be clipped or crushed. And again, just as with film, you want to be as generous to the "real" parts of your subject, letting the background fade out black, white, or whatever as appropriate.

    I'm sure that the RED compression algorithm works hard to allocate high-frequency detail to both high-contrast, but also the mid-dominant range of the image. With higher compression ratios, proper exposure of the subject (not necessarily an 18% gray card) becomes even more important.
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  3. #13 Gemini excepted 
    Senior Member Blair S. Paulsen's Avatar
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    Whatever ISO you set during capture will impact what you see on the monitor and what it looks like when you open the file in post. It has zero direct impact on actual captured values, but, if you use the monitor image to guide setting the stop and shutter, then it indirectly influences exposure choices. There is no technical difference between changing ISO on camera or in post. Until you create RGB imagery from the R3Ds they are merely a data set.

    CMOS CFA capture requires an algorithm to create an RGB image. This can be done internally, and on some cameras the RGB product is all you have access to. With RED R3Ds, the data set has a nominal RGB development instruction set in the sidecar RMD file. This facilitates opening and viewing an image, but until you "bake out" an RGB file it's just one potential version of what you captured.

    FWIW, my preference is to always set the ISO in camera to 800 so I have a consistent reference point during production. Phil has generated some charts showing how many stops are usable above and below middle grey at various ISO settings. As Les suggests, nothing beats doing your own tests. As Björn notes, there may be times when you want to capture more stops in the highlights or stay well clear of the noise floor - you don't need to change the ISO setting during capture to do that, but, as a practical matter, it might be easier to visualize if the monitor image is created at an intentionally chosen ISO.

    Cheers - #19
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  4. #14  
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    It's already been answered, but, changing the 'ISO' in-camera or in post is/does the same thing.
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  5. #15  
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    Thank you everybody.
    In post production I usually use the iso setting to dial in the image values to a kind of "mid" position on the waveform scope, and then use lift, gamma and gain to tweak the end and midpoint values. I find this a more practical way than only using lift, gamma and gain if the image is over- or underexposed, even if both ways presumably achieve the same result. I liked very much what Björn said about a volume control. I guess that "offset" could be used instead of iso, but I'm not sure.
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  6. #16  
    Member Mark A. Jaeger's Avatar
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    I have put together a YouTube tutorial on DSMC2 ISO. I was not happy with my understanding of how it worked so I researched and consulted with a helpful/knowledgeable GURU in the RED world (he reviewed my script). The tutorial is a little longer than I wanted but, if you hang in, it does provide answers to most of the questions posed above.

    You can find it at https://youtu.be/x1uAMYq7yQk.

    I hope it helps answer some of the questions that swirl about on this subject.
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  7. #17  
    Senior Member Alex Lubensky's Avatar
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    Quote Originally Posted by Andi Loor View Post
    As a former (analog) stills photographer I am used to the film stock having a light sensitivity by itself. When there was less light it was possible to underexpose the film by a few f stops and over-develop it, thereby getting an ok result although more grainy. The digital sensors of today are a different matter and I'm not fully comprehending how manipulating ISO in different ways is affecting the imagery. Also I have heard that different camera manufacturers use ISO differently.
    I understand we have the sensor and it's sensitivity and I believe changing ISO does alter the position of neutral grey, up or down towards the limitation of the dynamic range, so I believe changing the ISO actually decreases the dynamic range. What I have difficulty understanding is if there is any difference in changing the ISO in-camera or in the post production, as well as if there is a difference between changing the ISO in post or changing the exposure in the post. Is there a better or worse way to handle over or underexposed images in regard to this while correcting the levels in post?
    Some early video cameras had a +6 db switch, an electronic boost similar to over developing film. Do modern video cameras handle it the same way or are we just cranking it up in the post?

    I would be happy to get some more input in regard to this and to understand if different manufacturers handle ISO differently, and if so, how. Thank you.
    Tha basic principle was never changed - you can assume the "base" ISO of the camera as a film stock speed. Like, Alexa or most of Red Cameras have a base ISO of 800 - and you can relate to it as film stock base ISO, like Kodak Vision 3 has 500 ISO for example. Anything you change on post or in camera is like development - you push, or pull from that base setting. This is how most cinema cameras work - like Alexa, Venice, Reds etc. On film you also have a range of possible room to push or pull, which is determined by film stock possibilities - it's the same in digital. The only difference is - film is a substructional media, and loves highlights, while digital is an additive media, thus loves shadows more than highlights.

    On the other hand you have non-raw cameras, like Sony A7S, Canon 5D series, even C100 or c300 (mkI & II), FS-7 etc - which are relating to ISO change a tad differently. The thing is most RAW cameras (Alexa, Venice, all RED's) - record all the information that happen on sensor, that's why changing ISO shifts the DR within the sensor up or down without actually cutting off anything. Non-RAW cameras react on the sensor level in a same way, but when they record the image to the media they actually throw off some information that can not be jammed into the color space limits (be it log or rec709 lut). Film stock has 11-17 stops of DR depending on stock, while most digital cameras have something between 9-17 stops nowadays. BUT, the .REC709 color space has 5.2 stops of DR, which is also called SDR, which is what you're used to view on your TV or monitor, or tablet, or phone. And different camera manufacturers have different tricks to squeeze all the DR the camera has into the .REC709 color space. That's why cameras like Canon 5D, or Sony A7S look like changing ISO is changing a base stock. They just throw away all the information that they can't sqeeze into the codec, or sometimes just lack the DR on sensor level.
    Last edited by Alex Lubensky; 09-09-2020 at 09:44 AM.
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  8. #18  
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    Technically there is no difference to changing ISO in-post or in-camera, but the in-camera setting will be used for the physical exposure of your image on set (shutter, f-stop, nd, amount of light used)... So that's a massively important distinction.

    Similarly, while the DR doesn't change with ISO (with Dragon onward, anyway), the usable DR does. If you shoot at ISO 100 for cleanliness, but have to open to f1.4 to capture stops below mid-grey, you'll invariably blow more highlights needlessly/sooner. Conversely, if you rock f22 (or ND2.1) to capture all your highlights, thinking you can blast up to ISO6400 to get shadow detail back, you could just be pulling up the noise, making it more visible in important parts of the image (like the mids).

    Also worth nothing, RED's base ISO has never given an even over/under exposure. Alexa, Sony, BMD, most others are roughly even at base, but some RED's feel like they're weighted as much as ~2/3rd under neutral. With Skintone Highlight you get a stop more balance at the same ISO, however the sensors are noisier by at least a stop, so it kinda undermined the highlight advantage of it (IMO).

    (OT Rant: This kinda thing holds RED back, IMO. The less than ideal ISO/DR relationship, ISO settings not matching meters (until ISOCal2), different OLPFs making the ISO factor even more nuanced/finicky, etc, only served to undermine confidence and honestly didn't add enough benefit even when you knew what you were doing... But I digress...)
    Last edited by Mike P.; 09-09-2020 at 01:21 PM.
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  9.   This is the last RED TEAM post in this thread.   #19  
    Quote Originally Posted by Mark A. Jaeger View Post
    I have put together a YouTube tutorial on DSMC2 ISO. I was not happy with my understanding of how it worked so I researched and consulted with a helpful/knowledgeable GURU in the RED world (he reviewed my script). The tutorial is a little longer than I wanted but, if you hang in, it does provide answers to most of the questions posed above.

    You can find it at https://youtu.be/x1uAMYq7yQk.

    I hope it helps answer some of the questions that swirl about on this subject.
    Thanks for making the video Mark - you explain ISO very well and cover all the key points accurately.

    Only caveat is that Gemini has the two gain modes, so although ISO works just as Mark describes within a gain mode, changing gain mode does alter DR.

    Graeme
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  10. #20  
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    Graeme,
    Thanks. It is nice to have your support.

    To all,
    I made the video to set aside, for others, the lack of understanding and mis-conceptions that I had when I first started exercising my RED. I truly hope that, at least for some, I have elevated understanding. The present video now includes chapter marking such that, if you don't want to watch the whole thing, you can index to points of interest.

    Best regards, Mark
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