What is “ISO” on a digital camera?

Question

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?

Asked by mattdm

Answer

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 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.

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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.

Answered by lindes

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Why am I getting such high noise when using ISO 320/640?

Question

I'm taking photos at night with a GH1 at ISO 640 and getting a ton of noise in the dark areas.

I've seen a bunch of other peoples night time photos and they never have this much noise. I tried using a really long exposure of 25 seconds to compensate for the low light but there's still a ton of noise. Is this unavoidable or are there other settings I can use to get rid of the noise? Or am I supposed to de-noise all the images in post?

The first shot is right before the moon came out and the second shot is when the moon was fully out (its was around a 65% full moon that night). I also shot these in Aperture Priority mode if that makes any difference. Thanks for the help!!

EDIT: I was googling and I just came across Exposure to the right(ETTR). Could this be what I'm experiencing here? The images were underexposed and therefore lose a lot of values?

20mm, F11, ISO640, 25sec exposure

20mm, F6.3, ISO320, 15sec exposure

Answer

This question is probably best answered in two parts.

Part1:

You may need to increase the ISO to combat noise. This sounds counter intuitive due to common misconceptions regarding noise.

Noise is principally caused by lack of light. Lightsources emit photons randomly, the more photons you collect, the more the randomness averages out, leaving a smooth image. Fewer photons -> more random -> lots of grain.

All increasing the ISO does, is amplify the signal (and noise) you get from the sensor prior to digitization. It doesn't create noise, and as it amplifies the signal and noise by the same amount the signal to noise ratio remains the same. Needing to amplify the signal implies you have a weak (and hence noisy) signal to begin with, hence the association with high ISO and noise.

Noise is also introduced when trying to read the analogue signal. By amplifying before reading you add a small amount of read noise to a big signal, with little effect. By shooting at a lower ISO (keeping the other settings the same) you amplify less, to you add a small amount of read noise to a small signal, giving you a worse result by lowering ISO.

Part 2:

Reducing read noise by upping the ISO will help, but the primary problem you have is lack of light. You need to get more light down the lens, by either opening the aperture, or increasing the exposure time.

If increasing the exposure time/aperture causes other parts of the scene to be too bright (to the point where they show up pure white) then you have an additional problem that the dynamic range (the difference between the brightest and darkest parts) of the scene is too high, this can be fixed by using graduated filters, or by combining multiple exposures.

If you have a static(ish) scene I would recommend shooting several identical 25s exposures and then averaging them in software to reduce the noise. This is analogous to having one very long exposure, except without problems due to overexposure of certain areas. This is how most good DSLR astrophotography is done.

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Why do the 600D/7D sensors have more noise at high ISO than 5Dmk2 or 1Dmk4?

Question

Hi I'm wondering why the high ISO performance in a 5Dmk2 or 1Dmk4 is so much better than a 600D or 7d? I've been trying to photograph the milky way galaxy and when I use an ISO of 6400 I have an incredible amount of noise on the 600D, but I've seen the same ISO used with the 1Dmk4 and 5dmk2 to achieve wonderful photos without any apparent noise. I'd have to say the same for ISO 3200 which I can't get picturea without bad noise, ISO 1600 works but that causes noticeable star trails as the exposure becomes longer than the calculations I make using the declinations. Is this because of more pixels in the sensor size? Or do different cameras just have different ISO performance? Thanks for any insight.

Answer

The bigger the pixels, the less noise there is. This is a matter of physics. More light gets accumulated in each pixel and so it take more noise to appear significant.

The 600D and 7D have APS-C sensors which are small and have a high megapixels count. This makes their pixels comparatively smaller than the 5D Mark II which has a larger sensor and hence larger pixels since the resolution is not much higher than that of the 7D.

You can see the same thing in the Nikon range by comparing the D3X and D3S which are both full-frame cameras but one has twice the number of megapixels. The one with fewer megapixels shows less noise because each pixel is larger.

The extreme example of this is the poor performance of compact cameras which have tiny sensors. The last factor is technology because newer sensors show improvement over previous generation but physics cannot be beaten.

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Is there any noise difference between averaged and long exposure photos?

Question

Let's suppose I'm on a tripod, photographing a perfectly still scene (also dark) and I take these photos:

• 5 photos at ISO 3200 and 1s exposure
• 1 photo at ISO 100 and 5s exposure

There is a common thing between the items, and it's the total time used.

The EV of the first item is much higher, right? Now suppose I average the 5 photos at ISO 3200 to reduce noise, producing a single image.

After that, I take the ISO 100 photo and I adjust levels (which would increase noise) to reach the same EV of the blended photo, in a way if I look these 2 photos from far away they'd look the same.

Would the noise level be equal, comparing the blended photo and the levels adjusted photo?

I hope you understand my point.

EDIT

In response to drewbenn's commentary

Also, I don't think that blending the 5 photos will reduce noise the way you think it will

Blending photos reduce noise a lot, in fact here is an example:

I took 20 photos of a tree with: ISO 1600, F4.1 and 2s exp. The upper image is showing how much noise any of those images have. The lower is showing the result of averaging the 20 photos in one.

Sory for the bad focus.

As you can see, the noise gets almost completely deleted

EDIT2

For the ones who are asking, I used a very simply command of imagemagick to average the images:

convert [input1.JPG input2.JPG ...] -average output.JPG

If I have some time later, I'll try to conduct one of those experiments you're talking about. I guess there is no a static pattern and it'll vary on each camera.

EDIT3

I've also done a experiment a little more different:

This is the Scene:

And I've taken these set of photos (the aperture is always the same), I used manual mode.

• 01 @ ISO 100, 0.6s
• 02 @ ISO 200, 0.3s (averaged later)
• 04 @ ISO 400, 1/6s (averaged later)
• 08 @ ISO 800, 1/13s (averaged later)
• 16 @ ISO 1600, 1/25s (averaged later)

Each set has the exactly same EV, these are the results, in the same order:

It seems that a higher ISO, there is less noise but less details as well.

Answer

Provided your ISO100 image was not underexposed I wouldn't expect a noticeable reduction in noise (except maybe in the deep shadows) with the 5 1 second ISO1600 images blended together.

In the infamous other thread I demonstrated that a 1/30s ISO100 will contain more noise (lower signal to noise ratio) than a 1/30s ISO1600 image. Same amount if light but the higher ISO had less noise.

The reason for this was that the read noise is proportionally greater in the ISO100 image (as readout happens after amplification). In a "correctly" exposed ISO100 the read noise is so small compared to the signal that any reduction in read noise is probably not noticeable.

edit: just did the experiment

I shot one photo at ISO100 16 seconds, and 16 shots at ISO1600 but only 1 second. All images were well exposed. What follows are two crops, the top row is a single ISO1600 image, and the bottom two are the 16 ISO1600 images averaged in Photoshop, and the ISO100 image. I wont tell you which way round the bottom two are, to see if anyone can actually tell the difference - I certainly can't!

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What makes a good sample image for ISO comparison?

Question

To be more specific this image is always used by Digital Photography School for ISO comparison. What are those features that make this image suitable for ISO comparison.

Answer

In ISO comparisons you need to have a bit of everything, shadow, good range of mid tones, and some highlights plus a range of colours. This shot has all of that, which is why it's an ideal shot to analyse how well a camera deals with various ISO settings. Basically for any performance analysis in a single shot there needs to be as wide a range of colour, tone and detail as possible to judge how a camera deals with each of these factors so thats what makes images such as this very good for the job.

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What are the pros/cons of the high-ISO noise reduction feature on D90?

Question

So my D90 offers "High-ISO Noise Reduction". Sounds like a good thing, but obviously there must be a cost of some kind. The settings are "Off", "Low", "Normal", and "High", and the default is "Normal".

What are the pros and cons of using this feature? Why not always set it to "High"? Or, should I be turning it off?

Answer

The pro's are self-evident:

• Lower noise on high-iso

The biggest con:

• Loss of detail

The high-iso noise reduction might remove detail mistakenly. While newer camera algorithms have gotten better at it, it's still not fool proof.

The settings between Off, Low, Normal and High dictate the amount of tolerance used for the setting, which affects the amount of detail that might be lost.

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