Digital Photography ISO

Discussion in 'Articles of Interest' started by VidThreeNorth, Aug 8, 2018.

  1. VidThreeNorth

    VidThreeNorth TPF Noob!

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    This is something I have been struggling with, and after reading this article, I'm still struggling. I "grew up" in film and I understand film ISO. I also understand the basics of electronic sensors. So I should see a digital ISO and know what it is telling me, right? Well, I really don't. I just use it like I was using a film camera as best as I can.

    "You probably don't know what ISO means – and that's a problem" Published Aug 6, 2018 | Richard Butler


     
  2. jcdeboever

    jcdeboever TPF Supporters Supporting Member

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    Digital ISO is similar to film. It is gain.

    When you need to gain shutter speed to meet your requirement. If your subject is stationary and your ISO is 200, your lens focal length is 50mm, happy with aperture setting for depth of field, and your proper exposure calls for 1/125s shutter speed, then your good. Same scenario but your subject is moving and you want to freeze the movement and you know you need at least 1/500s to do so, you will need to increase your ISO 2 stops, 800 ISO in order to gain up to 1/500s for proper exposure.

    That's how I look at it. I use it to gain shutter speed for double the focal length of lens when hand holding or when a subject is moving and I want to isolate the subject and movement.
     
  3. Ysarex

    Ysarex Been spending a lot of time on here!

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    Richard Butler can be obtuse at times and hard to understand, but he tends to be accurate.

    What camera are you shooting? That will help a lot.

    Joe
     
  4. snowbear

    snowbear fuzzy-wuzzy Supporting Member

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    or an amplifier - basically the same thing.

    Oh, and for the record, ISO refers to the "International Organization for Standards," the acronym actually derived from isos (Greek for "equal").
     
  5. VidThreeNorth

    VidThreeNorth TPF Noob!

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    . . .

    Yup. That's how I look at it too -- because that is all that I know about what is actually happening.

    Hmm. Good questions. I guess for the last year I have mainly used the Yi-M1 for stills, though I used a Sony a5000 quite a bit as well, and my Panasonic GF-3, and occasionally, other stuff. For the coming year, I think I will use the Yi-M1, Sony a5000 and the Olympus OM-D EM10. Oddly, after suffering through all the early firmware versions of the Yi-M1, the last version 3.1 is actually quite good for my needs. I think it is one of the best setup for use with adapted manual lenses around. And they probably released that firmware after the last of the bodies was already sold to an end user. I think I respect that much of their effort.
     
  6. Designer

    Designer Been spending a lot of time on here!

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    No.

    Understandable.

    Nope.

    ISO in film is related to the sensitivity of the film, however; ISO in digital is actually applied gain. Sort of like turning up the volume control on a stereo. Notice; it is applied to the data AFTER the data has already been captured.

    This implies that it has absolutely nothing to do with exposure, which it doesn't. The purpose of ISO in your firmware is for the camera's display to enable it to produce a viewable image for you to evaluate composition, histogram, and focus.

    Therefore; ignore the "exposure triangle" meme, and consider your exposure to be regulated by your aperture and shutter speed. You can let the camera select the ISO by setting it to "auto", which should give you a viewable image.

    Variables that affect exposure
     
    Last edited: Aug 14, 2018 at 8:39 AM
  7. Ysarex

    Ysarex Been spending a lot of time on here!

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    Shadow Improvement versus ISO setting.

    The Sony and Olympus you noted are included in Bill's data base -- no Y1-M1. I added the Canon for comparison so you could see a situation where there's a big difference. Apart from the Olympus jump from base to ISO 400 both the Olympus and Sony tend toward ISO invariance. I believe I read that the Y1-M1 was built on a Sony sensor and so it would likewise tend toward ISO invariance.

    Raising ISO reduces dynamic range. Here's that chart: PDR versus ISO for the Sony and Olympus. The Olympus again show a bump between base ISO and ISO 400 that suggests the sensor may have dual impedance channels.

    DR dropping with increasing ISO is the opposite of film behavior and so the film analogy is weak. Film DR drops when it is pushed and that's a better analogy to digital ISO. Contrary to popular misunderstanding ISO does not increase the light sensitivity of the sensor. When circumstances require, raising the ISO reduces exposure to the sensor and we're simply left with less recorded data due to less exposure. ISO takes that less data and by one or another or combination of methods brightens it to the level required for a normal brightness photo.

    As Richard Butler explains the colloquial understanding of ISO's relationship with noise is fundamentally a misunderstanding. Noise in a photo can result from multiple sources some pretty esoteric like heat build up due to very long exposure. For most of us doing normal photography there are two noise sources in our photos; shot noise and read noise. The dominant noise source in modern cameras is shot noise and that is a function of exposure. Note for example the noise level in this photo taken at ISO 12,800: socks -- I exposed for minimal noise. The noise level is a function of the exposure and not the ISO 12,800 setting on the camera. Shot noise is in the light itself and the better the SNR (more exposure) the less noise and the worse the SNR (less exposure) the more noise.

    Read noise comes from the electronics in the camera and in the case of ISO the electronics that boost, gain, amplify, brighten, the sensor signal to the ADC. With each generation of new cameras we reduce or remove read noise from our systems (better engineering). That's what ISO invariance is all about. With todays modern cameras read noise is rapidly becoming a non-issue. Note that in the graph above I referenced an old Canon 5dIII to get a result that wasn't basically pretty flat. With read noise eliminated from our systems ISO can be thought of as only useful to the process of generating a camera JPEG of the required brightness (can't live without chimping!).

    Joe
     

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