How is the native ISO of a digital camera determined ?

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  • Simon Simpson
    Simon Simpson
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    Cameras, Lenses and Shooting Gear
    on: August 20, 2020 at 10:24 am

    How is the native ISO of a digital camera determined ?

    I would like to know, in terms of the principles, how the ISO standard for the ‘sensitivity’ of the sensor of a digital cameras is defined.  How is the native ISO of a sensor determined and measured ?</p>

    I understand how the ASA/DIN (and later ISO) of films is determined.  I understand that the sensor of a digital camera is essentially an analogue electronic device and so electronic norms may be applied as to how the native or base sensitivity of the sensor is determined.  But does this standard use the maximum saturation of the sensor as a starting point; or a nominal ‘mid-grey’; or a series of reference points – as with reversal film; or the noise floor; or the signal to noise ratio; or… ?

    I have searched the internet and it is telling that I have not yet found one website which explains this, although there are lots of websites “explaining what ISO is” i.e. the sensitivity of the camera (actually they mean the sensor) – along with quite a lot of misinformation.

    I am wondering whether one or more of the esteemed members of PhotoPXL might have the answer and would be able to explain it in non-mathematical terms ?
    It might be an interesting and illuminating topic for discussion…

    • This topic was modified 1 month ago by Christopher Sanderson. Reason: Removed html formatting. Please do not copy and paste pre- formatted text
    Simon Simpson
    Simon Simpson
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    Re: How is the native ISO of a digital camera determined ?
    Reply #1 on: August 21, 2020 at 4:57 am

    Chris, I pasted straight forward unformatted text into my post.  For some reason the forum software has recognised it as HTML formatted.  I can assure you that it was’t.  Perhaps this bears further investigation ?  Happy to assist.

    Andrew Rodney
    Andrew Rodney
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    Re: How is the native ISO of a digital camera determined ?
    Reply #2 on: August 21, 2020 at 12:47 pm

    See:

    https://www.dpreview.com/forums/post/63778640

    And do ping Iliah (author of RawDigger and FastRawViewer). You’ll not find a better resource than Iliah.

    http://www.libraw.org/

    Author: “Color Management for Photographers”

    Simon Simpson
    Simon Simpson
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    Re: How is the native ISO of a digital camera determined ?
    Reply #3 on: August 22, 2020 at 4:45 am

    Thank you, Andrew.

    Simon Simpson
    Simon Simpson
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    Posts: 8
    Re: How is the native ISO of a digital camera determined ?
    Reply #4 on: August 25, 2020 at 8:02 am

    Possible answers:
    It depends…
    ISO ratings in digital cameras may be an illusion…
    There is no ISO for digital cameras (possibly).

    This article explains it pretty well in terms most photographers can understand.
    https://www.fastrawviewer.com/blog/mystic-exposure-triangle
    (although it is about much more than the misconceived exposure triangle)

    Actually, there is a parallel with sound recording where the effective ISO, or ‘sensitivity’, is the gain control on the recorder. Normalisation, or line-up, is achieved using a tone at pre-defined level.

    However, ISO becomes relevant when using a light meter external to the camera; and for those of us using the Zone System.

    Andrew Rodney
    Andrew Rodney
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    Re: How is the native ISO of a digital camera determined ?
    Reply #5 on: August 25, 2020 at 8:55 am

    Setting ISO speed does not change the sensitivity of the camera sensor to light, like the volume control does not change the sensitivity of a radio, a good analogy previously provided here. In both cases the setting (ISO or volume) controls only the signal processing, while the input stage (sensor, antenna) provides the same input signal. That’s why when ISO setting is raised up, automatic exposure results in more noise. The automatic exposure in this case decreases the exposure (that is, the combination of aperture and shutter speed is set to allow less light captured by the sensor which is the only real example of exposure). Less exposure, less light, which then equals more noise.

    Author: “Color Management for Photographers”

    Simon Simpson
    Simon Simpson
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    Re: How is the native ISO of a digital camera determined ?
    Reply #6 on: August 25, 2020 at 9:35 am

    Agreed.

     

    If I understand the article correctly it’s probably a misconception to say that a digital camera has a base sensitivity in much the same way that it is a misconception to say that a sound recorder, digital or analogue, has a base sensitivity.

     

    It would seem, therefore, that the ISO settings on a digital camera are provided just for the convenience of the photographer; but is actually a signal gain device (analogue amplifier and perhaps some digital jiggery pokery).

     

    I now recall that when I worked for BBC TV, the analogue studio cameras were adjusted electronically (aperture and signal level) according to a display of the signal coming from them; and the portable Sony Beta cameras had a notional ISO of 100 but, as a BBC colleague pointed out, this was a only nominal value and not really technically ‘correct’ for an electronic device.  But it did enable an incident light meter to be used to balance lighting and determine the lens aperture.

    John Cornicello
    John Cornicello
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    Re: How is the native ISO of a digital camera determined ?
    Reply #7 on: August 29, 2020 at 3:13 pm

    What do you consider to be the practical implications for the working photographer when it comes to setting ISO? Is it different for various situations? Studio using strobes or LEDs? Concert photography? Landscapes?



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    Re: How is the native ISO of a digital camera determined ?
    Reply #8 on: August 29, 2020 at 5:49 pm

    John – back in the “old days” when film was used for all photography, the rule of thumb was that one should shoot with the slowest film (low ISO) one could reasonably expect to use.  Film speed was largely determined by the size of the silver halide grains in the film’s emulsion.  The finer the grain, the the lower the sensitivity with crisper and cleaner results.  Faster films had larger grain struction and grainy images were generally not as nice to look at as ones that had very fine grain.  Sometimes I would shoot a large grained (fast film) to add grittiness to an image.

    The same type of principle applies in the digital photography world.  The camera’s sensor at the lowest numeric ISO value will give you the cleanest results; maximum dynamic range, lowest level of digital noise, maximum colour bit depth.  As others have said, increasing the ISO does not change the sensor’s ability to capture data, but rather applies gain (amplification) to the signal, all of which results in a level of deterioration of the image output.  The closer you are to that base ISO setting, the more limited the “damage” from amplifying the signal.  The higher the setting, the higher the negative impact, to the point where the image may be so bad that it is of limited use.

    In the studio, I tend to shoot at base ISO as I can blast a very large amount of light at the subject using my studio flash and can generally live with the small lag was the flash capacitors charge up. If I am using continuous light (something I generally only use on inanimate objects), I use a sturdy tripod and the length of exposure is irrelevant.

    In landscape work, I generally shoot with a sturdy tripod and I can get away with relatively long exposures using a small enough aperture.  If I want to freeze motion of a fast moving object, I will give up a bit of the technical image quality as I can usually clean things up in post (some of the modern noise reduction software is amazing).

    Bottom line is, you the photographer are in charge of making the decisions on camera settings that will give you the image you can live with.  You have to understand the trade-off  you are willing to make between ISO, shutter speed and aperture.  I rarely use auto-ISO and generally the first thing I fix is my working ISO,  my shutter speed (blur or freeze motion) and aperture selection (determined by depth of field requirements) drive that decision.  It’s the photographer’s job to manage these trade-offs.

    Mike Nelson Pedde
    Mike Nelson Pedde
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    Re: How is the native ISO of a digital camera determined ?
    Reply #9 on: August 29, 2020 at 6:03 pm

    It’s the photographer’s job to manage these trade-offs.

    Indeed. To me, this is one of the qualities of being a ‘photographer’. Anyone can take a camera, push a button and take a photo. Birds have done it, monkeys have done it, even crabs have done it. A photographer is someone who makes images, and an important part of that is understanding one’s tools. But we’re getting away from the topic with that.

    Mike.

    _____
    Mike Nelson Pedde
    Victoria, BC
    https://www.wolfnowl.com/

    Andrew Rodney
    Andrew Rodney
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    Re: How is the native ISO of a digital camera determined ?
    Reply #10 on: August 29, 2020 at 6:22 pm

    Higher ISO can result in LESS noise, the idea that higher ISO always is more noisy is false:

    http://digitaldog.net/files/CU_ISO100-800.jpg

    Author: “Color Management for Photographers”

    Andrew Rodney
    Andrew Rodney
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    Re: How is the native ISO of a digital camera determined ?
    Reply #11 on: August 29, 2020 at 6:29 pm

    Higher ISO captures can produce less noise than lower ISO captures. The concept that higher ISO is always nosier is false.

    http://digitaldog.net/files/CU_ISO100-800.jpg

    Author: “Color Management for Photographers”

    • This reply was modified 3 weeks, 3 days ago by Andrew Rodney. Reason: Trying to update image link
    Andrew Rodney
    Andrew Rodney
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    Re: How is the native ISO of a digital camera determined ?
    Reply #12 on: August 29, 2020 at 6:32 pm

    And, why this site can’t accept images properly (or why I have to keep logging in even when using “Keep Logged in” must be a Safari issue? –)

    Author: “Color Management for Photographers”

    Kevin Raber
    Kevin Raber
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    Re: How is the native ISO of a digital camera determined ?
    Reply #13 on: August 29, 2020 at 11:25 pm

    Andrew, the image links must be https, it’s not a site thing it is a wordpress thing.  You can add an image as an attachment if it doesn’t have an https link.

    Kevin Raber
    CEO & Publisher of PhotoPXL.com and Rockhopperworkshops.com

    John Cornicello
    John Cornicello
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    Re: How is the native ISO of a digital camera determined ?
    Reply #14 on: August 30, 2020 at 12:30 am

    I’ve been shooting as a “pro” since 1979. Understand ASA/ISO and film grain. Also agree that exposure is shutter speed/aperture, independent of the ISO and that ISO doesn’t change the sensitivity of the sensor. The article referenced talks about the lightness value in post processing, but I don’t think it is saying to shoot at a low ISO at the shutter/aperture combo you want and then fix the brightness in post. In the studio I generally stay at ISO 100 and power the strobes up/down to give the brightness I want at the working f/stop I want (the flash duration becomes the effective shutter speed). I am trying to figure out if the article referenced is trying to say to do something different, or if it is only poking sticks at the “triangle” reference.

    Andrew’s comments and examples about higher ISOs sometimes being cleaner lead me to think I need to do more research testing to see where/how this comes into play. The article is not about creative control (shutter/motion blur, aperture/depth of field). It seems to be about getting the cleanest image.

     

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