Alley in Pisa, Italy

by Lars Kotthoff

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Can someone please explain to me what ISO means in the camera settings.

In an answer to a question about taking pictures at a dance party, I was told to use ISO 1600. What does that number really mean? When should I play with ISO settings, only on night pictures, or also during the day?

I see that its the sensitivy of the sensor. What I dont understand is: why the higher ISO, the more noise?, I am confused; if the sensor is more sensitive shouldn't it get more color or more quality?

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I see that its the sensitivy of the sensor. What I dont understand is, why the higher ISO the more noise?, I am confused, if the sensor is more sensitive shouldnt it get more color or more quality? –  L.V. Sharepoint Architect Apr 11 '13 at 11:06
    
Some of the earlier more technical questions (like What is ISO on a digital camera?) go into depth on this. –  mattdm Apr 11 '13 at 11:14
    
+1 for asking the 'simple' explanation. –  Regmi Apr 11 '13 at 18:44
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7 Answers 7

up vote 19 down vote accepted

Lets try this as a very different explanation:

Imagine that you have been asked to record a piano recital at the local school. It will occur in the auditorium, and it will be full of parents and friends to hear the work of the pianists.

You are not a studio technician, but are doing this as a favor to a friend. You bring your laptop, and you have an old microphone that came with it, the kind on a long stick. Its very cheap, but its all you have.

When you arrive, you are provided 3 locations to record. There is a microphone stand on stage, one on the front row, and one in the back of the auditorium. You can use any you like.

During a warm up, you try your microphone. You place the cheap mic on stage, and record. The cheap mic picks up some faint talking from the audience, but the piano music is clearly heard.

You move to the front row, and do a new recording. Here the cheap microphone picks up both piano and talking, both at nearly equal volumes.

On the back microphone stand, the cheap mic can barely hear the piano music over the low roar of talking.

Of course, in our example 'noise' is the talking, because, well, its noise. Its really what you do not want. If this same concert was a rock band, with the amps turned up to 11, you would not hear the noise because the signal is drowning out the noise of people talking (or shouting). This is the same in photography, where brightly lit scenes have little noise, as the light overpowers any electronic noise or stray light. But in a poorly lit scene, like our piano concert, the noise (talking) can be nearly as loud as the signal (piano).

Turning your ISO up is similar to moving the microphone location back: the scene is poorly lit (the music is farther away), and you are trying to get the best of the piano sound as you can, even though there is talking around you. Imagine that ONLY the back microphone stand is available, just like it may be low light and there is no way to improve the light (move to a forward microphone stand). In your case, you are stuck with your lens wide open and your shutter speed at the minimum you can use without making the scene blurry from shake and subject motion. This is the back mic stand. You make do with what you have, which is noise. The microphone stand locations are analogous to situations where you are left with only increasing ISO to take your shot, and while the noise in the auditorium is not really the same as noise seen in high ISO shots, it does help us think about the impact noise has on the system a bit easier.

Often with talk of ISO, you with hear discussion about more sensitive sensors that work better in low light, and exhibit low noise in high ISO settings. So, using our scenario, let's say that a friend offers their very expensive, but extremely good video microphone and breakout box, and shows you how to use it.

Then you try the expensive microphone in the same three locations. Here, on stage, the expensive mic picks up only music, with no talking to be heard. In the front row, the expensive microphone picks up the piano, and on really quiet passages, some faint talking. And finally, in the back of the auditorium, the expensive microphone reproduces the piano music very well, but there is noticeable talking heard.

Now, the expensive microphone is much better at isolating the piano music and rejecting the talking. Perhaps it has a design just for this, like a shotgun microphone. This is very much like a high end sensor in a high end camera: Nikon D800, D3, Canon 5D, 1D, all have very sensitive, noise rejecting sensors, and are very good at picking up the piano sound and rejecting the talking. Cheaper sensors are like our cheap microphone, and do worse on the back microphone stand location.

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I wish I could give you more points, :) excellent answer for a noob in photography –  L.V. Sharepoint Architect Apr 11 '13 at 12:45
    
Very, very nice answer! –  Chinmay Kanchi Apr 11 '13 at 12:51
    
Amps turned up to 11, +1 –  dpollitt Apr 12 '13 at 13:43
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On a digital camera, ISO is an increase in amplification of the analog signal from the sensor. This amplification has the effect of recording an image as if the sensor were more sensitive to light. This is why we commonly call it "sensitivity", but the sensor doesn't really get better; everything is just ramped up, including noise. When there isn't enough light, it turns out that at this low level is the best place to do this amplification rather than later, so don't be afraid of high ISO if you need it to get adequately short shutter speed.

Where does shutter speed come in? Simple. The exposure of a scene with a certain amount of light is the product of lens aperture (like f/2.8 or f/8), shutter speed, and ISO. You can get the same final brightness in an image by decreasing or increasing any of these by one "stop", as long as you change another factor to compensate. (What is one "stop"?) In automatic modes ("program modes", in the jargon), the camera will make this adjustment automatically. Try setting the ISO manually to different values and see what happens.

Increasing the ISO and decreasing shutter speed or aperture will generally result in a noisier image, so why would you want to do it? Well, changing the other factors have an effect too. If the aperture is wide (low f/numbers), it lets in a lot of light, but it's hard to get the whole scene in focus. And long shutter speeds also let in a lot of light, but above a fraction of a second any moving subject will be blurred. (More on this here; and note that for technical reasons due to small sensor size, changing aperture doesn't have much visible effect on a compact camera, so shutter speed is the factor you'll normally be most concerned about.)

ISO usually starts from a base of 100, and each stop is a doubling of that number — 100, 200, 400, 800, 1600, 3200, and so on. So, when you are using ISO 1600, you are increasing the amplification by four "stops from the base of 100. That means you can decrease your shutter speed correspondingly by four stops. Shutter stops are also doublings, so these four stops mean you could go from a probably-blurred ¹⁄₂₅th of a second to a snappy ¹⁄₄₀₀th, with the same exposure.

So, a higher ISO may be the best choice for a certain scene. This is usually the case in low light indoors or at night, but can be the case even in bright situations when you want a fast shutter speed.

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ISO is the sensitivity of the film/sensor.

If, in manual mode with everything else fixed, you take two shots, one at ISO 200 and another at ISO 400, the second picture will be exactly the same as the first but only twice as bright.

In a digital sensor, ISO is actually amplification, not sensitivity, and since noise is also amplified, high ISO pictures tend to have more digital noise than low ISO pictures.

A common use for high ISO is to compensate for faster shutter speed allowing you to take pictures handheld (or pictures of moving subjects) that you wouldn't be able to in low ISO but you pay for that ability with an increase in noise.

In bright light you generally want to use low ISO because that will give you less noise; in low light you want high ISO, because you can't take the picture otherwise; and in the middle you select some middle ISO which works for you.

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Originally ISO said something about the "speed" of the photographic film (the chemical emulsion). A low number (like ISO 100) meant that the film was "slow" - it needed lots of light to be correctly exposed - ie. large opening and/or long shutter speed. On the other hand, the chemical grains were very fine, so it seemed smooth and could be printed in large formats without seeming grainy. A high number (like ISO 1200) meant that the film was "fast" - it needed little light. The flip-side was that grains were rather large, giving the picture a more grainy appearance.

For digital cameras, the ISO-setting is the gain - how much amplification is used on the signal from the image-sensor. With high gain, you need less light - e.g. can use faster shutter-speeds - to get enough light on the sensor. As amplification also amplifies noise, a high amplification - ie. high ISO-value/high gain - also means that any noise becomes more apparent. Much noise, will give the pictures a "grainy" look - not too unlike the "grainy" look of fast (high ISO) film. It's a bit like using the contrast-control on your TV; by increasing the gain make what is barely lit (very deep grey) - but distinct from unlit (black) - seem like it's well lit. But as there really is very short "distance" between the black and very deep grey, any noise in the signal will become very visible with the high gain.

I should add that the sensitivity of the image-sensor is also very important. If the sensor can separate unlit from barely lit, then the ISO-setting (gain) may allow you to get what seem like a well lit picture. On the other hand, a less sensitive sensor may not be able to distinguish the unlit from the barely lit - it will register both as unlit (black)... and then no amount of amplification (after that sensor) will give you a good picture, because the sensor hasn't registered anything in the first place.

If you're photographing at a party - indoors, without flash, poorly lit room, people constantly moving (ie. high shutter-speed); then using a high ISO-setting will allow you to take good pictures, even with a small amount of light. On the other hand, there may be some noise in the pictures.

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What ISO does. The larger the ISO value, the brighter the image. There is already a tonne of technical answers available on the site so here is my personal approach to ISO.

When to change it. I shoot fully manual at events. ISO is the last thing I change and I would recommend others get into this habit as well. The bottom-line is, you want to keep it as small as possible.

I will only turn ISO above 800 if the aperture is wide open and I need a very sharp shot but my shutter speed is going below 60, which is about the slowest I can hand hold without motion blur (different people are stable at different shutter speeds).

Film photography. Film is very neglected on this tech oriented site, buts its worth noting that the noise caused by high ISOs on film is not the same as digital sensor noise. In film, shots from a high aperture roll appear grainy. It can complement rustly metals and other rugged raw scenes. But of course in film, one can only change the ISO/ASA once per roll. Post production workflow tools can simulate this grain too on digital photographs.

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ISO, simply put, is a measure of how sensitive the film is to light; how quickly it reacts when exposed to produce the image in the celluloid.

In a chemical film camera, that is the basic idea. Film with higher ISO speed reacts more quickly to less light, so that you can adjust things like the shutter speed and the aperture to get other desirable qualities of the shot without the resulting picture being underexposed.

With digital, we can simulate this change in film sensitivity given a sensor that has a fixed basic sensitivity to light, by adjusting the "gain" of the voltage naturally produced from each photocell of the sensor. This amplification can be calibrated to produce digital image values that look similarly "exposed" as various ISO chemical film speeds.

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I provide a layman's perspective of ISO on my photography page at Photography in Europe along with a general discussion of digital photography. ISO in film days indicated the "speed" of the film. It was also termed ASA. In the digital non-film world ISO applies to the "speed" of the sensor. Film ISO is fixed but can be modified during processing. Digital ISO can be varied through a wide range on most digital cameras.

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