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Given the same amount of exposure, sometimes you may prefer smaller or bigger aperture because you may want high or low depth of field.
Similarly, sometimes you may prefer slow or fast shutter speed, because you may want to freeze the action or you may want the blurred effect.

In case of ISO, is there ever a reason to prefer high ISO to low ISO , all other things being equal? (By all other things being equal, I mean exposure is the same i.e. image is properly exposed. Obviously changing the ISO means you will have to change the other 2 parameters to ensure same exposure.)

To clarify some confusion about what I mean, I will try to give a concrete example.

Let us say, you have five images of a well lit subject, shot with different apertures, all properly exposed (through adjusting the other two parameters). I can understand that one may choose the lowest aperture or the highest aperture image based on what kind of DOF he wants.

Similarly, if I have five properly exposed images with different shutter speeds, one can choose any of them based on whether they want blurry effect or freeze framed photo.

So, if I have five properly exposed images with different ISO settings, is there any reason one would not go for lowest one (apart from just the artistic/stylistic choice of someone who prefers higher noise in the image)?

My restrictions are deliberately designed to eliminate the situation where you need sufficient exposure and therefore HAVE to use high ISO. I'm not asking about when low light/moving subjects/potential camera movement with the lens already wide open conspire to prevent using lower ISO and getting proper exposure. I know, in those cases, you HAVE to use high ISO. I am asking about the cases where you do not HAVE to use high ISO.

Also, I found one other great answer by Matt Grum on this site which helped my understanding a lot. Thank you Saaru Lindestøkke for pointing me to that.

What is “ISO” on a digital camera?

The answer is in some places quite contrary to what is the common understanding about ISO settings, but if it is true (and from the huge upvotes in that answer, I assume it is true), then that is a very helpful answer.

Michael C
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silverrahul
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    Relevant: https://photo.stackexchange.com/q/35136/9161 and https://photo.stackexchange.com/q/6615/9161 – Saaru Lindestøkke Jun 18 '21 at 11:53
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    Your restrictions pretty much eliminate most of the valid reasons one would rather use higher ISO than lower ISO: when there is not sufficient light available to properly expose with a desired Tv (for freezing motion of a moving subject) or Av (to provide the desired DoF and also prevent motion blur). – Michael C Jun 18 '21 at 17:21
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    It seems like no one in the answers is really answering the question. I do know that there are some instances where I have preferred higher ISOs in nighttime shots as they give the photos a nice "grunge" or "old-school" look to them. However, you could argue that this is an effect that could be added in post and the lower-ISO photo is always better as it contains more information about the scene. But then again, sometimes we prefer doing our effects in-camera rather than leaving it for editing later on. – Skeleton Bow Jun 18 '21 at 18:36
  • @MichaelC My restrictions are deliberately designed to eliminate the situation where you need sufficient exposure and therefore HAVE to use high ISO. That is not what i am asking. I know, in those cases, you HAVE to use high ISO. I am asking about the cases where you do not HAVE to use high ISO. – silverrahul Jun 18 '21 at 19:48
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    @SkeletonBow Thank you, you seem to be the only one who understood the point of my question. It would be awesome if you elaborated your comment into an answer . Even though you did not completely answer it, you have understood exactly what i am asking – silverrahul Jun 18 '21 at 20:01
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    What is the real problem you're trying to solve? You've excluded from consideration every factor you could think of that anyone would use to choose an ISO setting. Anytime anyone mentions a new factor, you edit the question to exclude it. – xiota Jun 18 '21 at 21:06
  • Changing the aperture always changes the image; changing the time changes the image unless it is a still life. The image you want to create with these two "dials" determines the third "dial", the ISO setting, unless it is not available or too noisy, in which case you have to compromise your desires regarding the image. If the timings are irrelevant below a certain threshold then yes, use a slower ISO and the longest time that captures your scene according to your wishes. That should result in the best image quality. – Peter - Reinstate Monica Jun 18 '21 at 21:32
  • @Peter-ReinstateMonica " If the timings are irrelevant below a certain threshold then yes, use a slower ISO and the longest time that captures your scene according to your wishes. " Thank you. this is what i was asking for – silverrahul Jun 18 '21 at 22:31
  • @xiota i have tried to clarify the real problem i am trying to solve. judging from your response and most other responses , i understand that i have not been able to clearly convey what i wanted to ask. The edits to my question were , meant as clarifications to point out why those factors pointed out are not what i mean. To answer your question, just read the last part of question , with the concrete example of the 5 images. That is the real problem i am trying to solve. Hope this helps. Sorry if my question is not very clear. i tried to explain best i could – silverrahul Jun 18 '21 at 22:36
  • @silverrahul Please edit your question to add the specific photos you are reviewing. – xiota Jun 18 '21 at 22:39
  • I am not sure which constraint you are referring to ? For the 5 images, i am assuming that the higher ISO will have higher noise ( this is what i had understood is what happens with different ISOs ) – silverrahul Jun 18 '21 at 22:59
  • Also relevant: Should higher ISOs really be preferred (all other things being equal)? – BlueRaja - Danny Pflughoeft Jun 19 '21 at 02:22
  • For film, exposure = Av + Tv + film sensitivity (ISO or ASA or DIN). Film with higher ISO has more chemical transformation occurring for the same amount of light falling on it than less sensitive film does. For digital exposure = Av + Tv. ISO (analog amplification) is applied after the sensor has accumulated charges from the photons it captures. Increasing the ISO settings does not increase the number of photons captured by the sensor using the same Tv and Av with the same scene at the same brightness. – Michael C Jun 19 '21 at 07:36
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    @MichaelC Thank you. It seems i was under the common misconception which seems to be on so many sites that "Increasing ISO means increasing noise" . It seems the correct answer is "decreasing aperture and shutter time increases noise " . The misconception seems to arise because Increasing ISO does lead to decreasing aperture and shutter time in all the non-manual modes. – silverrahul Jun 19 '21 at 07:53
  • @silverrahul The question header invites responses concerning using higher ISO when low light/moving subjects/potential camera movement with the lens already wide open conspire to prevent using lower ISO and getting proper exposure. Perhaps you could rephrase it to indicate your intent to apply only to situations where longer exposure is not an issue? The most logical response to "Is there ever any reason to NOT prefer lower ISO?" is, "When you HAVE to use higher ISO in order to freeze action in low light!" – Michael C Jun 19 '21 at 08:00
  • @MichaelC Okay, i will do that. I have definitely realised that i did a woeful job of phrasing my question , which led to answers that are addressing something different than what i intendeed. I am thinking of how best to phrase my question, but everytime i try to edit it , i end up creating even more confusion. I initially thought about deleting the question altogether and maybe posting a new well phrased question, but so many people have worked hard to give answers, so i realised it would not be appropriate to do that. – silverrahul Jun 19 '21 at 08:05

11 Answers11

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Generally, the best image quality will be obtained at base ISO, which is usually the lowest ISO setting normally available. Some cameras let the user set an ISO value lower than the base ISO by enabling "expanded" ISO settings. In that case dynamic range is reduced.

See PetaPixel: Lower ISO Doesn’t Always Lead to Higher Quality Images,
Or this answer at Why is the lowest ISO always 100?

Orbit
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If you actually cannot choose from among several photos, your inability to choose means there is no significant difference among them, regardless of ISO. Just pick one randomly and move on.

All other things being equal, there is about a stop of ISO values that would result in correct exposure. Within that range, you can use a higher ISO if you want a brighter image. You might see improved shadow details, but you would need to take care not to blow highlights. Otherwise, you would need to change other settings in the exposure triangle. Choose settings that fit the lighting and subject.

Higher ISO is useful when:

  • You're using faster shutter speeds to freeze motion.
  • You're using smaller apertures to increase depth of field.
  • You're working in low light. You want to see more shadow detail.
  • You want to use a camera feature that requires higher ISO values. (Dynamic Range Expansion, increased noise reduction)

Lower ISO is useful when:

  • You're using long shutter speeds to allow motion blur, such as when photographing running water.
  • You're using large apertures to blur the background.
  • You're working in bright light. You want to avoid blowing highlights.

So here's a real example where it makes sense to use higher ISO settings.

  • Indoors. 1/30s. F1.4. ISO 100. Kids playing. If you have steady hands, absolutely nail exposure, and catch the kids in a still moment, maybe you'll get a couple clear shots out of a dozen.

  • Indoors. 1/120s. F4. ISO 3200. Kids playing. Faster shutter speed and greater DOF makes it more likely that you'll get the shots you want. The major criteria for picking keepers will be composition, facial expressions, etc, not whether you nailed focus or captured too much motion blur.

Everyone looking at grainy photos of kids playing will appreciate how cute the kids are. No one looking at blurry pictures of unrecognizable people will be impressed by how little noise there is.

See also:

In your hypothetical scenario, it's not necessarily the case that the photo taken with a higher ISO setting will have less noise than one taken with a lower setting.

  • Lower ISO settings will not reduce noise without sufficient lighting. An underexposed image that is pushed several stops in software will have noise, even though the ISO was set low. An image pushed in software may have the same or more noise as using the correct ISO setting, depending on camera.

  • Some cameras produce minimal levels of noise across a range of settings. There may be no practical difference among photos taken within that range.

  • Some cameras may produce better results with ISO values at some interval. ISO 640 may have less noise than ISO 100.

See also:

xiota
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  • The points you are raising is why i had added the clarification in my question and gave a concrete example of what is the situation i have in mind.

    I should not have said " all other things being equal " I meant exposure being equal, the other 2 parameters will obviously not be equal in order to maintain the proper exposure.

     – silverrahul Jun 18 '21 at 16:20
  • So, the baby example is irrelevant. Both the lower and higher ISOs do not have any blur, because it is a well lit subject. And difference in ISO has been compensated for by adjusting the other 2 parameters ( and the other parameters were required to adjust only a little bit, i am not talking about a situation where the other 2 parameters have been required to be adjusted to some unreasonable extreme amount. ) So, both the low and high ISO image are properly exposed, without any bluriness . – silverrahul Jun 18 '21 at 16:20
  • The 3 use cases of Higher ISO you gave i.e. fast shutter speed, small aperture and working in low light are all examples, where i HAVE to use higher ISO to get the proper exposure. There is no conflict between using high or low ISO in that case, because we need high ISO in those examples to get the proper exposures. I am talking about the example that i mentioned in the clarification of the question, where we have well lit , subject and we have 5 properly exposed photos with differing ISOs say 100,200,300,400 and 500 . – silverrahul Jun 18 '21 at 16:21
  • There is no bluriness etc. in any of those. So, in this case is their any reason to prefer the 500 ISO over the 100 ISO ? Because, the way i see it , if their is any noise, it will be more in the 500 ISO image. So, is their any reason to prefer the 500 ISO over the 100 ISO ? – silverrahul Jun 18 '21 at 16:21
  • At any given combination of aperture, shutter speed, and subject illumination, a digital camera's higher ISO setting should be expected to yield a picture which if if anything less grainy than a one at a lower ISO (the only danger would be blown highlights). The one direct advantage of using a lower ISO is that it allows one to use more light (wider aperture, longer exposure, or better-lit subject) without blowing highlights. – supercat Jun 19 '21 at 16:52
  • @supercat " At any given combination of aperture, shutter speed, and subject illumination, a digital camera's higher ISO setting should .......... " I am not talking about a given combination of aperture , shutter and subject illumination. I am talking about when all these factors are changing to compensate for the different ISOs – silverrahul Jun 19 '21 at 18:51
  • @silverrahul: I was responding to your comment about "lower ISO settings will not reduce noise without sufficient lighting". I think I would have said that essentially the only way a lower ISO can reduce noise is by allowing the camera to collect more light; if the camera's availability to collect light is maxed out (it's not possible to increase aperture, exposure time, or subject lighting) then reducing ISO can only make things worse. – supercat Jun 19 '21 at 20:12
  • @supercat “At any given combination of aperture, shutter speed, and subject illumination, a digital camera's higher ISO setting should be expected to yield a picture which if if anything less grainy than a one at a lower ISO” — This seems completely contrary to normal experience. You’re saying that I will get less noise by shooting ISO 3200 than by shooting ISO 100 (assuming a well-lit, completely stationary subject and proper compensation in shutter time to make sure neither shot is over- or underexposed)? This has never been true in my experience. – Janus Bahs Jacquet Jun 20 '21 at 14:28
  • @JanusBahsJacquet That is exactly what he's saying. You'll get LESS (or at worst NO MORE) noise by shooting ISO 3200 than by shooting ISO 100, PROVIDED THAT aperture and shutter speed r same for both cases , AND provided dat ISO 3200 doesn't blow out the highlights . This might be counter to your experience because in any other mode except manual, changing ISO from 100 to 3200 changes d other 2 parameters to compensate, in which case, ISO 3200 will have more noise , but that's because of the change in the other 2 parameters, not due to change in ISO per se. – silverrahul Jun 20 '21 at 15:06
  • @JanusBahsJacquet Read Matt Grum's excellent answer that's linked in the question – silverrahul Jun 20 '21 at 15:08
  • @silverrahul I virtually always shoot fully manual. If I leave aperture and shutter speed at the same values but change ISO from 100 to 3200, I will get significantly more noise. The 100 shot may be underexposed and the 3200 shot may be overexposed, but noise will absolutely be higher for 3200. If you look at DPReview reviews, this is also evident in their high ISO tests. – Janus Bahs Jacquet Jun 20 '21 at 17:22
  • No, you might be referring to blown highlights in ISO 3200. If you are talking just about noise, then that should not be the case. Check the side by side images in the link to Matt Grum's answer , link is in the question. can you give link for the DPReview reviews you are talking about ? – silverrahul Jun 20 '21 at 17:28
  • @JanusBahsJacquet You see more noise at ISO 3200 with the same aperture and shutter speed because you probably increase ISO in response to lower levels of light. Visible noise is a consequence of the signal-to-noise ratio. More light = more signal. – xiota Jun 21 '21 at 00:56
  • @xiota No, I am talking about taking two images right after each other of a stationary subject lit with lights. The subject, scene and light is exactly the same in both pictures, only the ISO is changed. // silverrahul: The images in Matt’s answer are both processed with auto-contrast, which means they are treated differently in post. You can easily see this by the fact that they are almost equally light, despite being very different ISOs. I’m not talking about blown highlights (you can’t see noise there anyway), but about areas that aren’t blown out. – Janus Bahs Jacquet Jun 21 '21 at 06:37
  • " I am talking about taking two images right after each other of a stationary subject lit with lights. The subject, scene and light is exactly the same in both pictures, only the ISO is changed. " Can you give links to these images ? Also links to the DPreview ISO tests you talked about ? The Images in matt's answer are treated differently, but that is just to make their brightness similar IN POST PROCESSING. That is after the images have been captured by camera. So , the noise in the images does not change because of that. It only makes the already existent noise more visible to naked eye – silverrahul Jun 21 '21 at 08:36
  • You want to use a camera feature that requires higher ISO values. (Dynamic Range Expansion, increased noise reduction) You NEVER want those features - those are features specifically designed to combat the IQ loss when you shoot at high ISO. You don't shoot at high ISO on purpose so that you get to use noise reduction - you only need noise reduction because you shot at high ISO in the first place. The only reason to go to higher ISO is if you don't have enough light to shoot what you want at lower ISO, and it absolutely always costs image quality to do it. – J... Jun 21 '21 at 14:22
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Apart from the obvious consideration that high ISOs will be preferred in situations of low available light, one further point not yet mentioned is that in the context of film, different emulsions with different ISO ratings have vastly different grain characteristics. Sometimes prominent grain (high ISO) is chosen for its artistic effect, even when a lower sensitivity emulsion could in principle have been used.

(In film, low and high ISO emulsions also tend to differ in other respects, such as contrast.)

In digital, the point is more or less moot, as "grain" can always be added in post-processing.

Kahovius
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  • I used to shoot 3200 b/w precisely for this reason. – Tetsujin Jun 18 '21 at 11:54
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    So,, as long as i am shooting only digital , lower ISO will always give better quality ( as long as i am keeping exposure correct by adjusting the other two parameters ) , right ? – silverrahul Jun 18 '21 at 12:30
  • Yes, if the exposure is the same and you apply the same post-processing to the raw image, then shooting at base ISO should give better quality. But see @xiota's answer, in particular, for all the complexities involved in this. – Kahovius Jun 18 '21 at 16:23
  • @silverrahul *Only* if there is sufficient light to give the image brightness you desire at the Tv and Av you desire. If you use low ISO and don't have enough light to properly expose for your desired Tv and Av, then you'll have to push exposure in post. That's usually noisier than shooting with a higher ISO to begin with. If you have to push exposure four stops in post because you shot at ISO 100 and the Tv and Av you wanted to use, you'll usually get less noise shooting with the same Tv and Av at ISO 1600. You'll also get more quantization errors by pushing exposure after ADC. – Michael C Jun 18 '21 at 17:51
  • @MichaelC If you don’t have enough light to properly expose for your desired Tv and Av, then by definition, you’re not “keeping exposure correct by adjusting the other two parameters”, which is the scenario the question explicitly discounts. – Janus Bahs Jacquet Jun 20 '21 at 14:23
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Yes, even with your constraints.

And let's eliminate "artistic" reasons of wanting more noise and restrict ourselves to technical reasons. I will also not discuss the "extended" ISO below the base, which effectively shoots at base ISO and pulls the image, which is covered in other answers.

You already know from the linked answer that it's always better to let more light in (up to a certain saturation point). So if you can afford to set the exposure (shutter and aperture) such that you just expose your sensor at the base ISO without blowing highlights, you should do exactly that.1

But doing this is a practical problem. In practice, you will probably blow some highlights, especially in high-contrast situations. And if you religiously avoid doing it, you will likely underexpose most of the frames, and will have to push them, increasing the noise. So here is the reason:

  1. Highlight recovery is better at higher ISO.

Say, if your sensor physically saturates at 2E6 photons (per pixel), and this forms the highlight (100%) at the base ISO (say 100), any extra light will be unrecoverable. Now if you double the ISO (to 200), your highlights become 1E6. The sensor can still linearly capture more, but the electronics and/or algorithms cut it off.

Now, depending on how the sensor and its electronics/algorithms are designed, it may leave much of the extra highlight information in the RAW file, making it recoverable by pulling it in post processing.

In my experience, Canon sensors and cameras benefit more from this. Some of them have a "dynamic range extension" mode called D+, which notably limits the minimum ISO at 200 (with the standard base being 100), and apparently leaves more headroom in highlights. Sony sensors, in contrast, don't tolerate blown highlights well, but instead don't mind pushing.

This works up to a point, and in practice may encourage setting the ISO a stop or two higher than the base, but not more.

I can think of another potential reason to raise ISO:

  1. Higher ISO may help to avoid shutter speeds prone to mirror slap effects (on DSLR cameras).

(Note: for this answer, we assume that shutter speed per se is not a constraint, which effectively means we are shooting from a tripod).

SLR cameras will invariably shake a bit due to the mirror slapping during the exposure. There is a range of shutter speeds most affected by this, about 1/1 to 1/100 s. If you find yourself within this range at the base ISO but could get out of it by raising ISO a stop, you may well get a sharper image despite slightly higher noise. On the other hand, you could just use mirror lock if you can afford it.

1 Note: the old Matt Grum's answer you refer to argues that you should choose higher ISO rather than underexpose the image and push it later. This is still generally true but less so nowadays with some modern sensors that digitize the signal right on the sensor. For them, pushing the image (from RAW, of course) and raising ISO is practically the same thing. In any case, for the purposes of my answer I assume that you can physically expose the sensor any way you like, including unlimited shutter speed.

Zeus
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  • Thank you for this response. It was gave some great nuance, and enlightened me with some great new information that other answers had not touched upon. I didnt understand all of it, but it gave me some good topics to look up , to understand the whole thing better – silverrahul Jun 21 '21 at 07:43
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To clarify: you can't have shutter speed and aperture the same and change only ISO. This will result over or underexposed image. Those three parameters depend on each other so you can increase ISO, but you need to close the aperture or increase the speed. The same is true about other combinations.

For me the main reason to set high ISO is to deal with low light. And because in some places you can't use tripod you should use high ISO or do not have the photo at all. Other case is when I need high shutter speed (but prefer to stay in aperture mode). One more case is if someone want to have more noise in to the image for artistic purpose.

Romeo Ninov
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While aperture and speed control the amount of light that reachs de the sensor, ISO only controls the amplification of the signal. So, in a world of perfect sensors there would be no difference between high and low ISO.

That's why with a different combination of aperture and speed you may get the same exposure, but different photos. You are literally changing the light: more light in less time, or less light for more time.

ISO, in other hand, does not affect the light reaching the sensor. It defines the amount of amplification of the signal before analog-to-digital convertion. So, ISO by it self does not change the photo, except for the noise.

Of course, if you trade ISO for other parameter, speed or aperture, you do change the picture.

vsis
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    I think this answers the OP's question best: the only effect of higher ISO by itself (you could compensate the higher ISO by putting an ND filter in front of the lens so the aperture and shutter speed are not affected) is more noise, so that's the only inherent reason for choosing a higher ISO if a lower ISO would also work. And any form of noise can always be added in post-processing, so if you like noise, there are better, more controlled, ways to add them to your photo in post. – Erwin Bolwidt Jun 18 '21 at 23:20
  • @ErwinBolwidt " the only effect of higher ISO by itself (...........) is more noise, so that's the only inherent reason for choosing a higher ISO if a lower ISO would also work. " By "that" being the only reason, what is the "that" you are referring to ? You do not mean " wanting the picture to have noise " , do you ? Your next sentence about adding noise in post-processing implies that is not a good reason for choosing higher ISO – silverrahul Jun 19 '21 at 05:40
  • @ErwinBolwidt Except it's not using higher ISO that results in noise, it's using the shorter Tv or narrower Av when using higher ISO that results in less signal (light) reaching the sensor, thus increasing the influence of noise in the S/N ratio. – Michael C Jun 19 '21 at 11:06
  • @vis Rather than a world of perfect sensors, what would be needed is circuitry between each of the sensor's photosites and the ADC convertor that produces zero read noise. There would still be Poisson distribution noise ("shot" noise) due to the random nature of distribution of photons at any given time within a light field of a specific intensity to deal with, and a stronger signal always suffers less "shot" noise than a weaker signal does. – Michael C Jun 19 '21 at 11:11
  • @MichaelC If Av/TV were not adjusted after choosing a higher ISO, and if the amount of incoming light would not overflow the sensor at the higher ISO, then there would still be extra noise because of the amplification of the signal. (and also higher values registered for the cells of the sensor array) – Erwin Bolwidt Jun 19 '21 at 12:40
  • @ErwinBolwidt Not usually, because analog amplification is done before the signal travels through the circuitry to the ADC that is the source of most 'dark current' noise. Analog amplification takes place before much of the noise is added to the signal. Mathematically increasing the numbers after ADC (such as in post processing raw files) will increase signal and noise at the same rate, but that's not what increasing ISO does. Poisson distribution noise will be the same in either case (same Tv and Av, different ISO), because it is a property of light, not of the camera. – Michael C Jun 19 '21 at 15:34
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    Not to mention that when we say more noise what we usually mean is a reduction in the S/N ratio. If both signal and noise are increased proportionally, the S/N ratio remains the same and the image does not appear to be "noisier" unless both signal and noise were below the black point prior to the increase. – Michael C Jun 19 '21 at 15:41
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    @MichaelC: Indeed, it's only fair to judge noise by comparing pictures that are adjusted in post-production to have the comparable lightness and contrast. Increasing ISO by a stop will cut by half the amount of contrast boost that will be needed in post-production. If it also increases the raw noise amplitude noise by 40%, that will mean that the noise amplitude in the adjusted higher-ISO picture will be 70% of what it was in a low-ISO picture that's adjusted to match it. The increase in raw noise will mean one only cuts noise by 30% instead of 50%, but 30% noise reduction is still a win. – supercat Jun 19 '21 at 20:16
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You probably want to avoid "lowest" ISO settings, if they're "extended"; i.e., achieved by digital pull-processing (increase the exposure, then lower it equivalently in post), because it reduces dynamic range. The ISO 50 setting on my Canon 5DMkII is done this way. I basically only use it if I'm willing to have decreased dynamic range and I need/forgot to bring a 1 stop ND filter with me.

inkista
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You may want to force the camera to use a different noise reduction algorithm.

Some camera use different noise reduction algorithms for different ISO settings. You can, for example, let the camera smooth out skin imperfections by taking a portrait with a higher ISO setting.

Of course, there is also post-processing software to do that job. However, this technique will be more pronounced with cheap cameras, where the owner possibly also has not the money to buy image processing software, or the time learn how to use it.

Klaws
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    " let the camera smooth out skin imperfections by taking a portrait with a higher ISO setting. " Do you mean using a higher ISO without changing aperture, shutter speed or do you mean using higher ISO, by compensating reciprocally by changing aperture and shutter speed ? – silverrahul Jun 20 '21 at 03:50
  • I mean by compensating in some other way. In the case of a portrait, you'd probably stick with the aperture and change shutter speed, or lighting. Well, in case of the cheap-camera-and-no-software scenario, the photographer is probably not having any adjustable lighting - except, maybe, for a flash. Of course, lighting can also be changed with an ND2 filter (<$2 including shipping from Chine), which can be held in front of the lens manually even if it has no filter attachment thread (a polarizer will work as well). – Klaws Jun 20 '21 at 07:33
  • How would increasing ISO and decreasing shutter speed, smooth out the skin ? – silverrahul Jun 20 '21 at 08:21
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    @silverrahul The (possibly cheap) camera will select a more aggressive noise reduction algorithm when a high ISO setting is selected. This will then not only reduce noise, but also fine details. Unfortunately, most comparisons of noise reduction algorithms are done for pretty high end cameras (which tend to be a less aggressive), while the effect will be more pronounced on some cheap small sensor cameras. – Klaws Jun 20 '21 at 10:15
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Yes…maybe. Confirm by testing your camera.

Each sensor has a minimum threshold before it registers a signal at a photosite. That quantity of photons is determined at the hardware level.

In the abstract, increasing ISO “pushes” the middle gray reading (or interpretation) closer to that minimum number of photons.

Imagine that middle gray was equal to the minimum value. All available dynamic range would be above middle gray. All the dynamic range would be in the highlights. All the detail would be there.

Conversely, low ISO moves middle gray “up” from the minimum number of photons the sensor can record. There will be more of the dynamic range available below middle gray and less above it… and that is usually what people want (favoring parts of the picture with too little light).

In terms of traditional wet photography, high ISO can be used somewhat like pull processing. Again on some cameras and confirm with testing.

The effects of high ISO on the distribution of dynamic range above and below middle gray are one of the reasons video gear is moving toward dual native ISO designs. The other reason is higher ISO’s come at the expense of dynamic range because the minimum number of photons the hardware requires does not change.

Again test your camera.

Bob Macaroni McStevens
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Physics provides some limits as to when you can change the ISO, all other things being equal. Because you targeted properly exposed shots, there's only three cases:

  • Change the ISO and shutter speed
  • Change the ISO and aperture
  • Change the ISO, shutter speed, and aperture.

I can't think of any unusual cases that show up in the third bullet which aren't covered by the first two, so we can just focus on them.

For "all things to be equal," if you're changing the shutter speed, that means the shutter speed didn't affect the shot. If there is any movement in the shot, choosing a different shutter speed will change the motion blur. So this indicates that you could change the ISO and shutter speed effectively in a still-life situation. A high ISO would permit a higher shutter speed. This could be convenient to get around camera shake.

The other approach would be to change the aperture. Lenses often have a "best" f-stop (I think it's typically around f-11). Higher than that, and you're using a small enough section of the lens that its hard to avoid imperfections. Lower than that, and you're using a large enough section of the lens that they have to have ground the lens correct in a global sense. This could lead you to pick an ISO that lets you choose a preferred f-stop.

I do believe those are really the only cases you could have "all else being equal." In other cases, you're meaningfully changing the picture's content, which is most likely covered under the "artistic intent" clause.

If you permit a real corner case, you might choose a higher ISO in response to some algorithm that is going to process your picture. There might be an algorithmic reason to want more noise, even if you could have had less.

And, never forget the trivial reason: to get a series of pictures which shows the effect of changing ISO! There's only one way to get that series, which is to change the ISO =)

Cort Ammon
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On film, the ISO value is a trade-off between sensitivity and coarseness. A film with a different ISO value had more of certain chemicals.

On digital cameras, the trade-off is between sensitivity and noise.

Of course, on a digital camera, the image captor is not replaced when you select a different ISO value. What is changed, is the threshold for detection of light. Apparantly, if you set the threshold low, the camera is less able to distinguish between light and noise. (I suppose that the noise is of electronic origin).

Co Stuifbergen
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    What is changed, is the threshold for detection of light. Apparantly, if you set the threshold low, the camera is less able to distinguish between light and noise. (I suppose that the noise is of electronic origin). This is not correct. Changing ISO does not change the sensor's sensitivity to light. What does change is the analog and/or digital amplification of the sensor's output voltage prior to analog-to-digital conversion (ADC). The noise is from several sources: thermal noise, pattern noise, quantization noise, amongst others. – scottbb Jun 19 '21 at 17:39
  • scottbb, thanks for the correction.

    I feel that the actual meaning of ISO in digital cameras is not well explained in the other answers, and I am glad that you have filled the gap.

     – Co Stuifbergen Sep 03 '21 at 14:12