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Thread: ISO Invariance

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  1. #21
    Super Moderator Kayaker72's Avatar
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    Dec 2008
    New Hampshire, USA
    First off, understanding all this does help. Right now, I think the
    have it figured out, likely because it helps that type of photography the most. At least three astrophotography blogs I found focused on where a camera becomes ISO invariant. They have focused on the ISO where upstream noise (generated by increasing ISO) is greater than downstream noise. Looking at charts, I would agree with you/dpreview concluded, for the 5DIV, it is about ISO400.

    But, dpreview bases all their "ISO invariance" tests on ISO 100, ignoring the fact that different cameras become invariant at different ISOs. Maybe they get it and are only concerned about base ISO. Then, some of the tests TN/Fstoppers did could be explained by a dual gain sensor (TN figured it out in the second video).

    So, sensors have different characteristics, from megapixels, to pixel size, etc. I think this is exploring a lesser known characteristic.

    Quote Originally Posted by HDNitehawk View Post
    Maybe Sony is doing less control than Canon in body and that is why Sony is more "ISO Invariant".

    From the link in your last post:

    "How does the 5D Mark IV stack up against some of its other peers? Its performance is much better than any other offering from Canon, but it's still not quite as ISO-invariant as the Nikon D810 or D750,"

    Perhaps Sony has given the illusion that its ability to handle DR is greater than it actually is.

    This quote, it really is another version of the DR discussion just said in a different way with different terms.

    The 5D Mark IV isn't entirely ISO-invariant: pushing an ISO 200 underexposed by 5 stops by 5 EV in post-processing yields slightly higher noise levels than a native ISO 6400 exposure. An ISO 100 exposure pushed 6 stops fares even worse. However, above ISO 400, the camera does, for the most part, exhibit ISO invariance, meaning that you could underexpose a traditional ISO 6400 exposure by 4 EV by shooting it at ISO 400 (while maintaining the shutter speed and aperture for ISO 6400), and then raise exposure 4 EV in post. This technique would afford you 4 EV of highlight headroom, with little to no noise cost, relative to shooting at ISO 6400.

    I've wondered about Sony as well. The whole "star eater" issue they had, it seemed that their RAW algorithm was looking for noise and crushing it to blacks and happened to eat a few stars in the process. Of course, this is an artificial/destructive way to create black/shadows with less noise. From what I've seen, the current algorithm is better, but I do wonder if it is still going on, just more refined. Then, I am still looking at this "dual gain" system of theirs. If ISO is post pixel, it almost has to be a digital manipulation.

    I've also wondered about Canon vs Sony's Bayer filters. How many photons are lost at the Bayer filter? I do not even see this discussed (other than by those praising Foveon sensors that do not have a Bayer filter). But, thinking about it, theoretically if you defined "Red," "Green" and "Blue" as very narrow, your colors would be more accurate, but you would have to block out more light. Define Red/Green/Blue more broadly, more photons make it past the Bayer filter, but your colors are likely less accurate. So, in DXOmark, if you just look at the measurements for an individual camera sensor you can see their estimated accuracy in color reproduction (ISO 17321). For the cameras I've looked at, Sony and Canon are pretty equal, and no one talks about this, making me think camera manufacturers handle this similarly, likely with widely defined colors, but still something I've wondered about considering Canon is generally praised for colors.

    All that said, I do think Sony decreased downstream noise with on chip ADCs decreasing the travel distance for the analog signal. I've also read Sony has more ADCs than Canon typically has, which would decrease heat, noise, etc. Then, in looking through Thom Hogan's articles yesterday, I found this. If this ever comes to market, this next generation of sensor will read "0" as pure white and "max" signal as pure black. What this does is shifts low end noise to the whites, where it will hardly be noticeable and blacks will have almost imperceptible noise. This is cool. Technically, it is either an odd manipulate of the pixel wells, or they are doing something like reading pixels lost by the photodiode and not pixels gained and held in the pixel well. Something, but still, I have to give credit where it is do, this, at a minimum, is a cool concept.

    Quote Originally Posted by Karsaa View Post
    I took 2 pictures last night one with 4s iso 6400 f/1.8 and other same but iso 800 will post later on so that i sync wb and tint for both images and then boost exposure +3 on the iso 800 so we can have some comparision that lvl. body is 5dIV
    Great, I've been thinking of doing some tests, so it will be great to see yours.

    Ultimately, this was about understanding the system for me. It is interesting that several astrophotography blogs were definitely on top of this subject, but I do wonder how much of a difference it will make.
    Last edited by Kayaker72; 04-11-2019 at 01:10 PM.

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