In situations where you aren't compelled to pick high ISO (low light/needing a high shutter speed), is there ever a reason to NOT prefer low ISO?

Question

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.

Answer 1

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?

Answer 2

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:

• What is the "exposure triangle"?
• What is the relationship between ISO, aperture, and shutter speed?

---

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:

• Photography Bay: Proof That Multiples of ISO 160 Work Best on Canon HDSLRs
• Is it better to shoot with a higher ISO, or use lower ISO and raise the exposure in post-processing?
• What is the significance of a camera's ISO Invariant point?

Answer 3

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.

Answer 4

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:

2. 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.

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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.

Answer 10

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 =)

Answer 11

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).