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Should an ISO Upper Limit be Set?

Discussion in 'Nikon DSLR Tips Forums' started by drr1531, Feb 11, 2019.

  1. In another thread discussion the ISO performance of the D500, I questioned the practice of using the upper ISO limit which many people seem to do. My own experience has been that underexposure(at any ISO setting) results in worse noise than properly exposing at higher ISO levels. So I asked, "what am I missing?"

    In response, Nick(Palouse) provided the following:
    Which is correct. Then:
    Which is true once ISO levels beyond the analog amplification range are reached and in-camera digital "gain" is used to further boost signal(i.e.increase ISO). If raising exposure further has to be done digitally then in-camera or during post simply depends on which software does a better job.

    OK, so point 1 is relatively clear. At ISO settings above the analog range it is really a choice of whether we think our PP software and skills do a better job than Nikon's in-camera digital signal boost. So personal preference.

    However, when shooting at ISO settings within the analog range, which is better, higher ISO or under exposing and correcting in post? When you under-expose at a given ISO(aka gain) setting you're reducing the signal strength without changing the (read)noise threshold. Which means signal to noise ratio(S/N) gets worse. So which is worse, reducing signal or increasing ISO/gain? Historically my own experience has been that properly exposing regardless of ISO setting produces best results. But I'd never gone out of my way to find any data to support one argument or the other.

    Well Bill Claff seems to have a lot of cred and his site, Photons to Photos, is packed with all sorts of technical information for those who care and insomnia cure for those who don't. But I've got one of those personalities and the curiosity was killing me so I spent some time there. And came across this article.

    Everybody get that? ETTR maximizes IQ :) 
    He goes on:
    To summarize, adjust ss and f-stop until you run out of room then crank up ISO to keep the histogram pushed to the right.

    Now here is where the upper ISO limit comes into play.
    Note this is a very old article that was based on testing with the Canon 1D3. The behavior is similar for most DSLRs but the top end of the analog ISO range varies by body, generally getting higher with newer generations. The analog limit for the 1D3 is actually ISO3200 but for that particular body there was very little to be gained with that last stop of ISO. Since then(2007) manufacturers have put a LOT of effort into improving low light performance so one can assume things have only gotten better since this work was done.

    So, bottom line, increasing ISO up to the limit of the maximum analog gain setting produces less noise than holding ISO down and under exposing. So logically if one chooses to set an upper limit to ISO, the maximum analog setting would make the most sense. But what is that setting? There's no single answer. It varies by camera body.

    I'm not sure how Bill Claff et al figure this out, but they have identified the points at which various camera bodies switch from analog gain/ISO to computational methods. Following are the max analog ISO settings for various Nikon bodies. Theoretically beyond these ISO values it is equivalent to raise exposure in camera or with PP.

    D300 = 1600
    D3 = 6400
    D3S = 12,800
    D7000= 1000
    D7200= 8000
    D4 = 25,600
    D4S = 12,800(interesting...)
    D810 = 2500 (this surprised me)
    D750 = 8000
    D500 = 51,200(wow!)
    D850 = 25,600
    D5 = 102,400
    Last edited: Feb 11, 2019
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  2. Dan, a good summary and a great challenge.

    I really like how this is worded:
    (underlining is my contribution).

    ETTR makes theoretical and practical sense, to me, and in landscape work it is very easy to do--I'm not usually rushed for time as you wildlife folks are!

    Like you, I wondered about the upper limits of ISO for normal shooting (shooting the once in a life time Bigfoot shot, and other such events are exceptions :D  , crank it up)
    Steve Perry provides this table that matches yours --except for a couple of cameras:

    Screen Shot 2019-02-11 at 17.49.33.jpg
    Subscribe to see EXIF info for this image (if available)

    More to puzzle about. I think I'll contact him for an explanation, re D850 and D500 especially. My experience with the 810 was high ISO was not its strength.

    This one I have to ruminate over a bit more: You wrote, "So, bottom line, increasing ISO up to the limit of the maximum analog gain setting produces less noise than holding ISO down and under exposing." I was of the opinion ( as stated in the other thread) that software could do a better and more selective job of pulling up shadows than the global approach that raised ISO would. I need to look at some of my recent shooting to be sure :cool: . But I do agree that the analog limit makes sense.

    The other thing to consider is the nature of the picture. One with lots of dark areas would generate more noise; while those with unimportant highlights would not suffer from a bit of highlight loss. Different strategies required, right?

    Anyway, thanks for adding to the discussion and bringing more logic and clarity to this.
  3. These topics are discussed in Marc Levoy's lectures at a fairly deep theoretical level. I will need to review that section before attempting to summarize his discussion here. But I'm pretty sure he said to avoid going into the digital amplification regime.
  4. It looks like Steve picked the ISO value where Claff starts to indicate "scaling"(which he does not explain) but still within the indicated analog portion of the curve. Presumably "scaling" in the analog region indicates some level of computational manipulation of the analog gain. From a practical standpoint the ISO values in question make it pretty much irrelevant for the vast majority of shooters.
  5. Ann_JS


    Feb 18, 2015
    New York State
    What remains clear to me that a bit of testing with our own cameras will be much more useful in enabling us to get the best possible performance from our equipment (and to find the limits to which we can push it when we need to!) than wasting much time on the theoretical charts produced by the various Measurebators.

    Worth noticing is that the ISO 102,400 which is shown in some charts as "Maximum Analog ISO" for a D5 is clearly shown as the upper limit for ISO settings on the camera itself with the higher ISO speeds above that clearly stated to be "Extended" and delineated as a "High #" value.

    There are also settings for "Low ISO" (below the Base 100 ISO) where some sort of de-amplification appears to be used.

    I don't need charts to tell me what is clearly marked on the camera and which was clearly stated in Nikon's published specifications.
  6. No question we should all run our own tests to understand the equipment that we work with. But measurebating has its uses for some of us. Sometimes it is helpful to validate one's subjective observations. Or to make one aware of confirmation bias/wishful thinking. Though some of us(many of us) make up our minds with very little and/or poor information and thereafter don't want to be confused by facts.

    That is the case for the D5. Not all of the Nikon bodies are that way.
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  7. I only read the title and glanced at the rest....I'm not interested in how the sausage is made but

    putting a limit on iso makes no sense as you are going to screw up something, SS or exposure or both, depending on what mode you are shooting...the whole point of auto iso is to let the camera select the iso needed for a perfect exposure (what the camera thinks is perfect)
  8. A wise person once told me that "you can't fix blurry".

    Sure I try to keep the ISO low and the histogram pushed to the right, but at the end of the day, you need enough shutterspeed at a given focal length to keep the photo sharp. These ISO charts are nice, but you can't fix blurry.
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