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IQ v. ISO & Bracketing.....Revisited

Discussion in 'Panasonic Cameras' started by Tom Swaman, Sep 17, 2011.

  1. Tom Swaman

    Tom Swaman Mu-43 Veteran

    Yes, I realize that exposure in digital photography can be complicated. Before posting this thread, I reread most of the posts here on the subject plus other material. I also believe I understand exposure control and sensor versus color sensitivity fairly well. However, I have a few questions which I hope are of interest to several of our members.

    Because I am using a GH2, I shall keep my remarks confined to the GH2 and its system.

    1) I understand and I accept that the best RAW image on a GH2 may be generated at ISO = 160. I understand that ISO = 320 is nearly as noise free as ISO = 160, but not quite as good. What I do not understand and question is why ISO values between ISO = 160 and ISO = 320 are so much worse than either of the two ISO values of 160 or 320? I thank anyone for explaining this to me.

    2) If I am shooting in aperture priority at or above say 1/60th second exposure and my ISO is set at 160, what difference in IQ results and why do these results occur if the resulting exposure is in fractional stops as long as the ISO value is not changed? Please know that I am not referring to differences generated by shutter speeed aberrations, but rather to differences in file noise.

    3) If I am shooting in shutter priority at or above say 1/60th second exposure and my ISO is set at 160, what difference in IQ results and why do these results occur if the resulting exposure is in fractional stops as long as the ISO value is not changed? Please know that I am not referring to differences generated by aperture aberrations, but rather to differences in file noise.

    4) Given that optimum IQ and lowest possible noise result at whole number multiples of a camera sensors base ISO value, does this mean that auto bracketing should only best be used in full stop increments and any needed image exposure adjustments be made in PP? Please explain if you are able. I am also assuming when asking this question that it is implicit that overexposure is to be avoided as a primary objective.

    5) Which is preferred with respect to IQ and noise reduction, avoiding shooting at itermediate ISO values or shooting at intermediate ISO values and PP with noise reduction software?

    6) Has anyone seen a good, readable treatise on optimal digital sensor exposure control other than the several links posted on this site?

    7) Finally, is the use of exposure bracketing as taught with film photography for optimizing exposure now only replaced by full stop bracketing for dynamic range control in digital photography?

    Here is a great big thank you to those who can help me and others understand these seemingly complex issues. To those who can help and do, please also accept an extra bucket of photons for your efforts.

    With appreciation,
  2. meyerweb

    meyerweb Mu-43 Hall of Famer Subscribing Member

    Sep 5, 2011
    Assuming we're talking RAW:

    As regards questions 2 and 3, I believe the answer is "none." If the ISO is constant, and the amount of light reaching the sensor is constant, so is noise. The sensor doesn't know or care if the light got there at 1/60 and f4 or 1/45 and f4.5 (or whatever the equivalent is) or 1/52nd at f 4.1.

    I'm assuming all the shutter speeds are fairly close. If you're talking about the difference between 1/500 and 5 seconds, then there are obvious differences, but not between 1/60 and 1/55th or 1/40th.

    Aside from that, can you point to any documentation that there are LARGE differences in IQ between the full stop multiples of ISO and intermediate values? Not that I've run any controlled tests, but I certainly haven't seen any dramatic differences in noise at 160, 200, or 320. In fact, noise is pretty much not an issue at any of those ISOs.

    In answer to 5, as a rule I'd say it's better to avoid noise reduction in PP unless it's really necessary. NR will inevitably cause some loss of resolution, although again, at normal viewing sizes and low ISO's neither the noise nor the lowered resolution will really be noticeable.

    If you're talking JPEG, then all bets are off. You probably lose more quality in the JPEG conversion than you do changing ISO, and most JPEG engines in cameras apply some NR by default, even at the lowest setting.
  3. Tom Swaman

    Tom Swaman Mu-43 Veteran

    Meyerweb, thanks. I am talking only RAW.

    I think I was unclear in my questions thanks to your answers. I do know your answes are correct. I only mentioned 1/60th second as this is an intermittent shutter speed which has little known effect on noise and IQ such as 1/2 second where the shutter speed is well known to affect noise and IQ.

    So, let me ask it this way, why is IQ and noise better at say ISO = 320 than it is at ISO say 210 or 260? I do inderstand the sensor whole number ISO issue, but I so not understand the intermediates stop ISO impact on IQ.

    I hope this helps.

    Best regards,
  4. ~tc~

    ~tc~ Mu-43 Hall of Famer

    Oct 22, 2010
    Houston, TX
    My understanding is that the intermediate ISO stops are interpolated. The gain on the sensor is not actually changed, the image is essentially push-processed, and - just like in post if you expose to the left - the shadow noise becomes exaggerated. Canon does the same thing and has the same issues, I'm sure there's a ton of write-ups out there for them
    • Like Like x 3
  5. Tom Swaman

    Tom Swaman Mu-43 Veteran


    Thanks much. Your excellent, direct explanation makes a whole lot of sense to say nothing about being totally logical.

    Have a great week.

    Best regards,
  6. In another thread we touched on the native ISO of the G3 (assumed to be the minimum 160) and I asked what the camera did when you set the ISO step to 1 stop which should presumably use multiples of the native ISO. It was reported that when the G3 was setup like this it chose ISOs of 160, 200, 400, 800, etc. NOT multiples of 160. Maybe tc or another G3 user could confirm this. I believe the GH2 also uses ISO 160 as a minimum.
  7. GaryAyala

    GaryAyala Mu-43 Legend

    Jan 2, 2011
    ~TC~ basically nailed the question ... but (the big but), to elaborate ...

    Every camera system has "Native ISO", which I think you are calling minimum. Native ISO's, IIRC, are derived from analog gain not digital exposure compensation. In Canon the native ISO's are 100, 200, 400, 800, 16,000, 32,000 et cetera.

    Typically, analog gain will give better dynamic range than digital exposure compensation while digital exposure compensation will deliver better/less noise.

    As an example ... say ISO 125, is ISO 100 with a 1/3 stop digital push. While ISO 160 is actually ISO 200 with a 1/3 stop pull down. This pull brings down the exposure of the entire image, and hides much of the noise that would be visible at the next higher ISO. ISO 160 is the cleanest because it is the native ISO 200 with a 1/3 stop digital exposure pull, yielding even less noise.

    All ISO's incrementally one step higher than a Native ISO are pushed. All ISO's incrementally one step lower than a Native ISO are pulled.

    So which ISO settings are best? It all depends. Non-Native ISO's "pull down" ISO's are less noisy, but at a price. That price is decreased dynamic range.

    Because ISO 320 is actually ISO 400 pulled 1/3 of a stop, that means that the highlights are going to clip at exactly the same point as they would at ISO 400. The 1/3 stop pull is just making that point 1/3 stop darker than pure white. The entire image at ISO 320 is 1/3 stop darker (and may be less noisy) than the image at ISO 400, so the blacks lose detail 1/3 stop sooner, but you don't get that 1/3 stop back at the highlight end of the range -- it's still gone. Therefore, at ISO 320 you're losing a net 1/3 stop from the total usable dynamic range that you would have if you were shooting at ISO 400.

    This is how Canon works and I suspect it is similar to the µ4/3 sensors as well.


    PS- I've often seen "Native ISO" used to describe the ISO where no manipulation occurs (no analog gain or digital exposure compensation).
    • Like Like x 3
  8. Tom Swaman

    Tom Swaman Mu-43 Veteran


    Thanks for your excellent amplification. I could be wrong, but somehow I recall reading that ISO = 320 is the native ISO for the Panny GH2. I wish I knew for certain.

    Best regards,
  9. ~tc~

    ~tc~ Mu-43 Hall of Famer

    Oct 22, 2010
    Houston, TX
    Yes, that is correct
    • Like Like x 1
  10. diselgl

    diselgl Mu-43 Regular

    Jan 7, 2014
    Have you read Bob DiNatale's article on getting the optimum digital exposure? Recently I've been experimenting with his technique and so far I'm getting positive result - low noise, good dr and good detail in my last few images.
  11. Ulfric M Douglas

    Ulfric M Douglas Mu-43 Hall of Famer

    Mar 6, 2010
    Tom I think you are basing your 'problem' on generalities,
    whereas your GH2 has a unique sensor (the first, the G5/G6 are admittedly tweaked versions)
    I think you would have fun (and be useful) doing a series of ISO-step noise tests with strict light and aperture to see where your ideal ISO choices are (and how shutterspeed is irrelevant)
    The last test of this type I did was back in the olden days with an Olympus e-600.
    It doesn't take long and with that sensor the results were plain as day.
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