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What is "ISO" in general, and how is the scale defined?

How does the ISO scale for film speed differ from ISO sensitivity as used in digital cameras?

Is lower ISO always better?

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I realized we have great questions/answers on aperture, decent ones on shutter speeds, and nothing which really gets to the heart of this third exposure factor. So, here we go. :) –  mattdm Jan 8 '11 at 18:18
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There's a fourth exposure factor: Amount of light. ;) –  lindes Jan 8 '11 at 18:41
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That's a matter of where you place your terms. :) For any given scene, there is a certain Exposure Value which will record an image with a certain average brightness value. By adjusting aperture, exposure time, or sensitivity, you change the exposure value, and for a constant scene that changes the "exposure" of the resulting output image. By changing the lighting, you change the scene itself, meaning that a different exposure value will be required to record the same brightness in the resulting image. –  mattdm Jan 8 '11 at 18:51
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Traditionally, EV is just Av × Tv. With sensor gain under easy immediate control, these days it's effectively: Av × Tv × Sv = EV. Then, EV / lighting = final exposure. –  mattdm Jan 8 '11 at 18:54
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See photo.stackexchange.com/q/2946/378 for a discussion of how digital cameras implement ISO. –  Evan Krall Jan 8 '11 at 22:12
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4 Answers 4

up vote 69 down vote accepted

In photography, ISO generally refers to a measure of "Film Speed", which I use including reference to digital sensor sensitivity.

In short, the actual letters ISO are a name for the International Organization for Standardization (not, officially, an acronym -- more information here), and in photography it refers to the ISO 12232:2006 standard and other related standards: ISO 12232:1998, ISO 5800:1987, ISO 6:1993 and ISO 2240:2003. (Links on the Film Speed page.) Film historically has also used the ASA and DIN standards, the former using the same numbering system, and DIN using an entirely different scale.

The standards for film and digital are technically different (in ways I haven't investigated closely enough to report fully on), but generally they're similar enough that for practical purposes, they're essentially the same (notwithstanding Reciprocity Failure, which many films are quite prone to, though digital generally is not). So if you measure an exposure with your digital camera, you could use that exposure with a film of the same rating as the ISO setting you used in the digital camera, and expect to get a similar exposure (unless the shutter speed is long or short enough for Reciprocity Failure to kick in for the film) (also assuming similar equipment otherwise -- no differences in filters, etc.).

In both digital and film, a higher number indicates greater sensitivity. A number twice as high is twice as sensitive (e.g. 200 is twice as sensitive as 100, 400 twice as sensitive as 200, etc.). So, when shooting in relatively low light, and wanting a relatively-fast (e.g. fast enough to stop motion) shutter speed, a higher sensitivity rating will be essential (so no, lower is not always better!)

In digital cameras (and similarly but differently in film), higher ISO ratings tend to create noise (the related effect in film is increased graininess). So, while it's not always better (that depends on what you're going for), lower ISO ratings are always (or at least almost always) lower in noise, which may be desirable. (In the case of low light photography where shutter speed is not a concern, long exposures mixed with lower ISO ratings will create a "better" image -- though it's conceivable that some may like the effects of the noise; certainly there's appeal at times to film grain.)

As for how the scale is defined, it is based on measurements of an image produced based on a certain scenario of illumination. The details are complex, so I'll leave the details as an exercise for the reader. A lay summary is that (for digital) it's a measure of how quickly the digital sensor becomes "saturated" with light. (For film, the process is related but different.)

In summary: higher ISO is more sensitive but noisier (but not necessarily worse), digital and film rated for the same ISO (or ASA for the film) will have similar sensitivity, and the scale is based on how quickly an image will become "saturated" given a certain amount of illumination.


NOTE: I'm gearing up to do some experimentation related to the controversial answer from Matt Grum. Hopefully, my results will create a nice clear noise-free answer to the important point he brings up: that a high-ISO image with a low amount of light will be less noisy than a low-ISO image with the same amount of light getting to the sensor, which is later amplified in post-processing. More to come, hopefully in... EDIT: Well, I've failed to make this happen for a while now. I may still do it at some point. In the mean time, I'll also point to this article that talks about comparisons of native versus non-native ISO values, and amounts of noise in them, which, while the article doesn't exactly say so, I think is probably directly related to exactly this question.

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+1 because it actually answers the question. –  Evan Krall Jan 8 '11 at 22:11
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+1 because of the same reason @Evan Krall said –  Wayne Jan 9 '11 at 1:36
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I never meant to be controversial, this answer contains lots of useful info, I just wanted to let people know that dopping ISO doesn't guarantee lower noise, and provide an example. I thought it about time that someone stand up for Captain ISO, in his relentless struggle against the evil Doctor Read Noise, for which he receives no recognition, all whilst Mr. Dim Indoor Lighting gets away scot free! –  Matt Grum Jan 10 '11 at 19:59
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ISO ratings can be stated either on a linear scale (the old ASA rating style) or on the logarithmic scale (DIN). Until relatively recently, you couldn't use the ISO name alongside the stated rating if you didn't state the values on both scales, even if the contrast curves and development were exactly in accordance with the ISO spec. At some point, somebody (or a committee of somebodies) finally decided that juggling two interchangeable ratings wasn't doing anything for anybody (except for the Deutsches Institut für Normung), required too many LCD characters to display, etc. –  user2719 Feb 8 '11 at 17:02
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ISO is effectively a sensitivity of the sensor, whether it be film or digital. In theory, ISO for a digital camera should be the same as for a film camera.

The ISO on film is determined by the grain size of the chemical. What this will mean is that the resolution will be better with a lower ISO film. Also, because a film grain is all or nothing, this will lead to some spottiness, or noise.

The way that ISO is achieved in a digital camera can help to understand why it is important, and why using a higher ISO isn't always a good thing. I don't know the way it was achieved in film, so I can't really speak to it, but I imagine it's using somewhat similar principals.

First of all, Matt did an excellent job explaining the different sources of noise. What happens in a camera actually comes down to the following.

  1. Light hits the sensor.
  2. The signal on the sensor is passed through a gain circuit. The amount of gain depends on the ISO setting being used.
  3. The signal value is then ran through an A2D converter. What this will do is map the amplified signal into a range from 0 to 255 (Or higher, depending on the number of bits in the converter), a digital signal.

The camera then may remove some of the basic sources of noise, especially the "Dark current" noise. That's the signal if you were to expose complete blackness.

So what actually happens when you turn up the ISO? There's basically two paths, which I'll cover below.

If the ISO is increased, and the shutter speed correspondingly decreased, then the signal level going into the A2D converter remains the same. However, the amount of signal on the sensor is decreased. That means that any noise which is sensor depended will in effect be amplified. This includes things such as Shot Noise. Some other forms of noise won't be amplified.

The second option is if the original image was underexposed, but the ISO brings it to being correctly exposed. It is possible to contrast enhance the underexposed image to have the same max/min as the correctly exposed image. The signal coming out of the A2D converter will effectively have a smaller range. This will leave only a few light levels, leading to a much noisier looking image than the correctly exposed image.

A few other interesting things. Sensors tend to bleed if they are saturated to neighboring pixels. If an image is increased in ISO and decreased in shutter speed, then it will not be saturated on the FPA itself, removing some artifacts. This can lead to somewhat better performance when saturated.

In general, high ISO on a digital camera will have less noise then the same ISO on film, due to a much better way to achieve high ISO.

Hopefully this wasn't too technical, but I'm just excited to share my knowledge of Electrical Engineering with the community:-) Let me know if you have any more questions, and I'll try and explain it even better.

To answer the second question, I'd say that lower ISO isn't always better, but in general, it's best to use the lowest ISO you can. If you are shooting indoors, for example, a higher ISO will allow you to use a shorter exposure time, leading to less motion blur. Usually I use the lowest ISO that will allow me to not be over the hand-limit of 1/lens_length shutter speed. But there are always exceptions.

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It should be noted that the analog signal will not necessarily be mapped to an 8-bit image. When working with RAW, image bit depth is usually 12-16 bits, with a very few medium format cameras offering full 24-bit RAW format. The number of digital levels could be anything from 255 (8bit), 4096 (12bit), 16384 (14bit), 65536 (16bit), or in the case of 24-bit MF, 16777216! –  jrista Jan 9 '11 at 5:54
    
Are there really any MF cameras offering 24-bit digitisation? 16.78 million is many many times more than the number of actual photons hitting each pixel (around 85k)! Therefore the extra bits can only be encoding noise. –  Matt Grum Jan 9 '11 at 14:28
    
@jrista: Thanks, I should have included that... –  PearsonArtPhoto Jan 9 '11 at 14:36
    
@Matt Grum I'm not so sure about that. Remember, those bits have to encode not only photon intensity, but also wavelength. I understand the 24 bits to encode all three color channels. (Please let me know if that's incorrect.) That's only 8 bits per channel, covering (therefore) only 8 EV. At the low end the discretization is awful: the jump from 1 to 2 is a full EV. Thus, we could expect dramatic improvement in shadow detail if we could get beyond a mere 24 bits. –  whuber Jan 9 '11 at 17:28
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The ADC (analogue to digital converter) only encodes intensity, not wavelength as no bayer interpolation has happened at the time the analogue signal is digitised. I was thinking that the 24 bits were for colour output at a later stage of the image pipeline but that's only 8 bits per pixel, which is surely too low for a MF camera –  Matt Grum Jan 9 '11 at 18:02
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Is lower ISO always better?

No!

For a fixed amount of light coming into the camera, lowering the ISO will not result in a reduction of noise. The only way to reduce noise is to combine lowing the ISO with letting in more light by opening the aperture of leaving the shutter open longer.

If the amount of light you can let in is limited (you have hit the max aperture and can't use a longer shutter speed without introducing blur) then using the highest ISO possible (without clipping highlights) will lead to the lowest noise in the image. This is going to seem counter-intuitive and invite downvotes (seven so far!), so please let me explain (see my response to comments at the end).

High ISO values don't cause noise, lack of light causes noise. The reason people associate high ISOs with noise is because when you increase the ISO setting in shutter or aperture priority mode, the camera either closes the aperture or increases the shutter speed to compensate, both of which decrease the amount of light coming into the camera.

Here's an explaination of why low light results in noise:

One important source of noise in images arises from the random nature of light and is called photon noise, or shot noise. Photons are emitted randomly from lightsources. If you collect a lot of photons the randomness averages out and you get almost exactly the same number emitted in each direction. If you collect much fewer photons the number collected in neighbouring pixels (that should see the same colour) can differ giving brightness variations referred to as noise or grain. This is how lack of light leads to noise. See

http://en.wikipedia.org/wiki/Shot_noise

Another source of noise is read noise. Read noise happens when the analogue charges on the sensor are digitised (read out). Read noise is approximately constant with respect to the number of photons captured. Increasing ISO amplifies the signal, and thus the photon noise, but the read noise stays the same.

If you use a lower ISO (with the same amount of light coming into your camera) you will get an underexposed image and when you brighten it in post you will amplify both the photon noise and the read noise. Your total noise will then be higher.

Here is an example of this effect in practice:

These images were shot with identical settings bar ISO, and processed in exactly the same way. The bottom one is clearly noisier, despite being shot at ISO100

If you were to plot noise vs ISO for every pair of shutter-speed/aperture, you'd find noise always increases (sometimes only slightly) as ISO decreases. Therefore I don't see how you can argue that high ISO results in noisier images, given that when all else is equal lower ISOs give more noise!

TL;DR

To minimise noise, get as much light as you possibly can down the lens (since noise is mainly causes by low light levels) using the aperture and shutter speed settings.

And then whilst keeping these settings constant, increase ISO as far as you can (without clipping highlights) as this will reduce the proportion of the signal that is read noise.


Response to comments

Sorry, but your explanation doesn't change the fact that your basic statement is wrong. What you normally want to use is the lowest ISO that will give proper exposure with a usable combination of shutter speed and aperture

The method you use is of no real consequence, when you think about it using auto-mode and picking the ISO value that results in the shutter/aperture being as long/wide as you can get away with is equivalent to just setting the shutter/aperture to be as long/wide as possible in the first place and then setting the ISO to obtain the correct exposure.

However, I wasn't suggesting a shooting methodology, the point I was making was, to minimise noise, you want to let in as much light as possible, given that you have achieved this with shutter/aperture/flash or whatever, the ISO should be as high as possible (before highlights clip), as anything else will result in underexposure and thus more read noise.

if you actually do change your shutter speed and/or aperture (following the law of reciprocity, as one would normally do), then guess what: you get more photons on the sensor for the same "exposure" (call it EV as adjusted for the ISO in question)! So, less noise!

Yes but you can't increase the ISO and at the same time change settings to let in less light and then conclude you have more noise because you're using a higher ISO. Especially when you can show that if you increase the ISO on it's own you decrease noise!

It's like buying a car with a smaller engine in order to reduce fuel consumption, driving everywhere with your foot flat to the floor and concluding that small engines increase fuel consumption! Change one variable at a time.

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That is, assuming, all other things are equal. Pushing the exposure in post processing will expose the noise, but if that ISO 100 shot had a longer exposure so that it was correctly exposed, the noise difference may not be so evident. Also, newer sensors make this a little less painful. :) –  John Cavan Jan 8 '11 at 20:02
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@John When making comparisons it makes sense to keep the other variables the same, no? Anyway the point I'm making is if the ISO100 shot had been two seconds it would have been less noisy, but that would be due to the extra light, not the lower ISO, because when the amount of light was the same the lower ISO did not result in less noise. –  Matt Grum Jan 8 '11 at 20:23
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-1. Sorry, but your explanation doesn't change the fact that your basic statement is wrong. What you normally want to use is the lowest ISO that will give proper exposure with a usable combination of shutter speed and aperture. Underexposure (regardless of ISO) will lead to noise problems, but the problem is the underexposure, not the low ISO. –  Jerry Coffin Jan 9 '11 at 4:12
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Matt, it may be that everyone is misunderstanding what you are trying to explain. I just tried a couple example shots, and ISO 100 always beats out ISO 1600 when both settings use the correct shutter & aperture to produce a correct exposure. I've created an example shot to try and demonstrate both points of view. Please see my answer for the example shot, exposure and post processing details, and further discussion. Perhaps we can get everyone on the same page. –  jrista Jan 9 '11 at 23:44
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I think, what I take from your answer and comments so far, is that when you are trying to use a specific shutter and aperture, if using a low ISO results in an underexposed image, and using a higher ISO results in a properly exposed image, using the higher ISO is better. Additionally, if you pick a specific shutter and aperture, pushing the ISO to its highest without clipping highlights, then reducing exposure in post-processing, will result in the clearest image with the lowest noise for the shutter and aperture you chose to shoot at. If I understand correctly, then I totally agree! –  jrista Jan 10 '11 at 0:25
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Regarding the statement:

Is lower ISO always better?

There seem to be a variety of opinions on this topic, and while they may seem mutually exclusive, I am not certain that is the case. There is no cut and dry "Yes, X ISO setting is always better." I think which is better is very dependent on context...on what it is you are trying to shoot, and what kind of light is available.

The two points of view expressed so fare include:

  • Use the highest ISO possible to maximize saturation (and therefor minimize noise) without clipping highlights.
  • Use the lowest ISO possible to minimize noise while using the correct shutter and aperture to achieve a proper exposure.

The general consensus is that opting for the lowest ISO possible to achieve a proper exposure is the best approach. That statement is heavily laden with hidden meaning, however, as the lowest ISO possible may not necessarily be ISO 100. You may be forced to use a higher shutter speed or a narrower aperture to achieve the necessary creative effects you desire, forcing you to use a higher ISO to maintain proper exposure. You may also face issues with available light and reach the limits of your lens (i.e. maximum aperture) and be forced to use a higher ISO to achieve a correct exposure. I think that is the point Matt Grum has been trying to make.

Barring any specific creative needs (i.e. action-freezing shutter speed or large DOF via small aperture), the lowest ISO setting with the necessary shutter and aperture to produce a "correct" exposure (i.e. an exposure that is neither over or under exposed, or if you follow ETTR, one that has not blown out any highlights) is, still, the best practice. This can be easily demonstrated, as the sample shots below show. This set of examples is shot with several things in mind:

  1. There is no requirement for a short shutter speed, so it could be as long as necessary to get a correct exposure.
  2. There is no requirement for depth of field, so the widest aperture will do.
  3. The available light is fixed, and can not be changed.
  4. A tripod and cable release will be used to eliminate any camera shake.

alt text

The clearest shot is, as you can see, the ISO 100 shot. At maximum aperture, ISO 100 required a 0.6 second exposure time. This is pretty long, but since there was no motion, a lengthy exposure is not an issue. The ISO 1600 shot is still properly exposed, and used a shutter speed exactly four stops faster than the ISO 100 shot. Despite being properly exposed, there is obviously additional noise. The final shot was another ISO 100 shot at the same settings as the ISO 1600 shot, with the exposure value adjusted by +4EV in Lightroom. The noise level in this shot is obviously FAR higher than the other two since it has been digitally enhanced.

Given these examples, we can come to a few logical conclusions:

  • Using the minimum ISO for scenes with no motion when shooting with a tripod will produce the lowest noise possible.
    • Common for landscapes and still life scenes.
    • Uncommon for sports, wildlife, bird, or any other action photography.
    • Depends for things like portrait shots, however since you can usually control the amount of available light for portraits, a low ISO should usually be possible.
  • Using the minimum ISO for a scene that allows you to expose correctly will minimize noise, but may not be the lowest possible noise.
    • If you need to freeze action with a high shutter speed, or increase your DoF with a narrow aperture, you may need to increase ISO to maintain exposure.
  • Shooting with the lowest ISO regardless of the situation is a bad idea.
    • This will often result in under exposure (possibly severe) when available light is not optimal.
    • This will result in considerably more noise when exposure is corrected in post processing.

I think a good way to learn what ISO settings are best for various scenes and lighting is to set your ISO to Auto, use manual mode, take a few shots, and review the results. The camera's automatic exposure metering will always try to create a "proper" exposure, and when you have a scene with a broad range of tones, it should choose the right setting most of the time. You can also try manually setting a higher or lower ISO than the one the camera chooses automatically, and re-take a shot to see the results. Outside of maybe landscape and still life photography, you probably won't find a single "correct" or "best" ISO setting. However, I do believe the general rule of thumb will always be:

Use the lowest ISO you can while maintaining a proper exposure for the type of shot you are making.

For still life, that will probably always be the minimum native ISO (not using any kind of ISO expansion). For landscape, that will probably be lower ISO's, such as 100 or 200. For action photography, including sports, wildlife, birds, kids, etc., the lowest ISO may change for every shot, and may vary from ISO 200 through ISO 3200 or beyond, and it will be very dependent upon available light. Lots of light will allow you to use lower ISO, less light will dictate higher ISO. Regardless of what ISO you use for action shots, another good rule of thumb is:

Its always better to actually get a shot than miss one because you don't like the camera settings necessary to capture it.

Even if you have to use ISO 3200 to get decent indoor sports shots with your f/1.4 lens, at least you'll get the shots. Those ISO 3200 shots will have lower noise than the ISO 1600 shots that you underexpose then correct via post processing, as shown by the (rather extreme) example above. De-noising algorithms are also pretty advanced these days, and can greatly reduce the noise level of a high ISO shot to more acceptable levels. That again makes using a higher ISO that will ensure a correct exposure a better option than a lower ISO that will likely underexpose and require post-process correction.

EDIT:

Further investigations to cover Matt's assertions about ISO have lead to another sample image. The image below is split into four bands, two representing exposure following Matt's methodology, and two representing exposure where ISO is kept as low as possible. The assertion was that when trying to expose a scene, you set aperture and shutter then use the highest ISO possible without blowing out highlights, and correct exposure via post processing by reducing EV, to produce an image with the lowest noise. This contradicts the normal assertion that one should use the lowest ISO and adjust aperture and/or shutter to achieve a correct exposure to maintain the lowest noise possible.

Noise Test

The first and third bands in the image above were shot by choosing a specific shutter speed and aperture, then upping the ISO as far as possible without clipping highlights. The second and fourth bands in the image above were shot by choosing ISO 100 and a specific aperture, and adjusting shutter speed to achieve a correct exposure (no ETTR.) Both images were corrected in post production by using Lightroom's "Auto Tone" feature, which caused a reduction in the high ISO image's exposure by a little bit, and pretty much left the ISO 100 image the same.

There is a difference of three stops ISO between these two shots, and the increased noise level of the ISO 800 image is very clear in the shades. In the midtones, there is some observed increase in noise in the ISO 800 image over the ISO 100 image. In all tonal levels, fine detail has been lost to one degree or another in the ISO 800 image vs. the ISO 100 image. This can be observed by examining the remote control in its cradle, the fronts of the palm, and in any shadow tones. In the mid tones and highlights, noise levels are not specifically high enough to cause any significant degradation in print. The noise level in midtones and shadows is, however, likely to intrude onto fine details, such as one might pick up with macro photography or any photos that push the limits of resolution of a lens or sensor.

While negative exposure compensation in post processing has indeed reduced the noise level of the high ISO image to more acceptable levels, there is no question that using the lowest ISO possible will result in lower noise than even post-process negative exposure compensation of a high ISO image.

The real question is, is it always appropriate to use the lowest ISO possible? If you can expose the scene you are trying to capture without any undesirable side effects, such as blurring, underexposure, etc. then choose the lowest real ISO you can (expanded ISO settings usually achieve a lower ISO setting through digital, rather than analog, means...so using ISO 50 Expansion for example, should be avoided.) If you can not expose the scene you are trying to capture without any undesirable side effects, which might be the case when photographing sports or wildlife, photographing concerts, or doing much of any kind of indoor photography involving any action, then increasing the ISO to the minimum acceptable level that will allow you to expose your scene properly (i.e. eliminate motion blur, expose at the right level, etc.) should be chosen.

If you have the headroom, overexposing by choosing the maximum ISO you can without clipping highlights, and applying some negative exposure compensation in post processing, can help mitigate the effects of noise of very high ISO, and will be a better choice than choosing an ISO that is too low, then applying some positive exposure compensation in post processing (which will just heighten the effects of noise.)

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I think you've misrepsented my point of view by saying Use the highest ISO possible to maximize saturation (and therefor minimize noise) without clipping highlights. my point of view is: To minimise noise, get as much light as you possibly can down the lens using the aperture and shutter speed settings. And then increase ISO as far as you can (without clipping highlights). I don't know how I can state that any clearer, suggestions would be welcome –  Matt Grum Jan 10 '11 at 0:18
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Your example shows three shots. Going from bad to good in terms of noise you've got Low ISO, fast shutter - very noisy, High ISO, fast shutter - noisy, Low ISO, slow shutter, clean. Given that the only consistent difference between the noisy and clean shots is the shutter speed, doesn't it seem logical to conclude that it is the shutter speed, not ISO100 that has lead to a cleaner image? This also fits with the theory with respect to photon noise. –  Matt Grum Jan 10 '11 at 0:24
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btw +1 for taking the time to investigate this rather than just dismissing it! I think an important point which I failed to mention is that an advantage of using a lower ISO is that you can let in more light before you clip highlights, thus enabling you to reduce noise (for landscapes etc.) but I still claim noise is proportional to the amount of light you let in, not the ISO. –  Matt Grum Jan 10 '11 at 0:28
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@Matt: I think you just nailed it all on the head: Noise is proportional to the amount of light you let in! Now that you've stated that, I entirely understand your perspective here. I think what I stated is correct, if more round about, in that if you have no option but to expose with "less light", using a higher ISO will produce a less noisy picture. I will have to experiment with pushing ISO to its highest without clipping, and reduce exposure during post-processing, and see the results. I am still not sure if that would produce an image with less noise than the lowest ISO setting possible. –  jrista Jan 10 '11 at 0:40
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@jrista That's essentially it. The other key thing is that while photon noise is proportional to how much light you let in, the insidious read noise can cause overall noise to increase when you drop ISO! For proof of this look at your second and third pictures, the amount of light coming in is the same but the ISO100 pane is clearly noisier! What you've said isn't wrong you're just approaching from a different angle. I hope both answers will contribute to people's understanding of the subject. –  Matt Grum Jan 10 '11 at 0:51
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