Besides affecting exposure, ISO also determines how much "noise" (digital term) or "grain" (film term) will be present in an image.
Back in one of the early posts on exposure, I mentioned that only aperture and shutter speed actually control how much light makes it into the camera, while sensitivity (a.k.a. ISO or film speed) determines how the camera responds to that amount of light. This is an important fact we'll need to remember as we discuss noise and grain.
Let's start by talking about noise in a digital image. In order to get a decent explanation, we'll need to say a little something about how the image sensor inside your digital camera actually works. Metaphorically speaking, the image sensor is like a bunch of tiny light-catching measuring cups arranged side by side (just image a field full of Pyrex). These metaphorical measuring cups, called "photosites", each measure the amount of light they receive and convert this into one pixel in the final image. If one measuring cup received a lot of light and is mostly "full", this translates into a bright pixel. If another measuring cup received only a little light and is mostly empty, this translates into a dark pixel. Ok, with me so far?
So, these measuring cups ("photosites") have a certain capacity, that is, it takes a specific amount of light to fill them up half-way, or all-the-way, etc. This is like the "native ISO" of the camera, i.e. the default sensitivity of the image sensor. This is normally the lowest ISO setting available on your camera and is also where you will get the highest image quality. The image below was taken at ISO 100, the "native ISO" of my camera. This is only a very small part of a picture of my dining room, enlarged to show the smooth texture and lack of noise provided by a low ISO setting.
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So what happens when you increase the ISO? We haven't actually increased the amount of light coming into the camera (only aperture and shutter speed control that) so how does a digital camera cope?
Let's go back to the measuring cup metaphor. If only a small amount of light is coming in the camera, the measuring cups (photosites) are barely getting full at all. At a low ISO setting, the camera would look at these mostly "empty" photosites and render the overall image too dark. When you increase the ISO, the camera tries to amplify the amount of light recorded by each photosite to make the image brighter.
As an analogy, imagine each of your measuring cups is a 2-cup size and they all have a very small amount of water (light) inside, let's say between 1 - 4 teaspoons. If you wanted to increase the amount of water in each cup, but do it equally so they are all still the same in proportion to each other, you might try to double the amount of water in each cup.
The problem is, it's hard to accurately measure small amounts of water in a big cup, that's not really where they are at their best. Is it 1 teaspoon in this measuring cup? 1.5? 2? You could guess the first time, and be pretty accurate. However, this level of error gets multiplied if you are trying to really increase the amount of water, say 4 times, or 16 times. This is the same problem (metaphorically) that your camera runs into at high ISO, and the mistakes it makes appear as noise.
The shot below was taken at ISO 3200. The graininess and colorful speckles are noise.
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Ok, sorry this post is so long, but a quick word about film. For film, sensitivity is normally called "film speed" and is an inherent property of the film that cannot be changed in-camera. Also, film works differently than digital and is made up of chemical crystals instead of electrical sensors. When enough light hits the chemical crystals, they begin bursting and create dark areas on the negative, which will translate into bright areas in the final image.
For films with low film speeds, these crystals are small and hard to burst, resulting in a fine-grained film that has high image quality but low sensitivity to light. For films with high film speeds, the crystals are large, easy to pop, but create a coarse grain that degrades the overall quality of the image. Some people find the look of grainy film appealing, and use this affect for artistic purposes (thus the "film grain" filter in Photoshop). I am not one of those people, but I thought I'd mention it just in case you are.
ISO = done! Next up, exposure mode!
