Question from a beginner photographer?

ISO is the sensitivity of the sensor to light. 100 ISO is 1/2 as sensitive as 200, which is 1/2 as sensitive as 400. As you move up in ISO, you get more noise in the image, so you want to watch how high your push it.

The basic unit of light is a stop, and every time you go up or down one stop, you double or halve the light or light sensitivity of the sensor.

So going from 1/30 to 1/60 will halve the time that the sensor will be exposed to the light, going from f/5.6 to f/8 will halve the light that enters the lens, and going from ISO 200 to 100 will halve the sensitivity of the sensor. All of these will produce the exact same change in exposure, but will cause different effects in the image. Increasing the shutter speed will help to stop motion, changing the aperture will increase the depth of field of the image, and changing the ISO will decrease noise.

So if you want to shoot sports, you need a fast shutter speed and you generally want a decent depth of field to get more than just one player in focus. If the light is not great, you need to increase your ISO to compensate for less light getting through the lens and then hitting the sensor for less time.

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The site below may be of help to you learning the basics. Scroll down to What IS ISO?

ISO is essentially a measurement of how sensitive the sensor (or film, if shooting film) is to light. The higher the ISO number, the more light the sensor or film can gather (this isn't automatically good, but we don't have to go into that yet).

Most cameras have a "base ISO" of 100. Some have a lower base, some a higher base, but for most of them, 100 is the lowest. The ISO doubles continually until you reach the camera's maximum ISO. On some cameras this is not so high (1600) and on others it's crazy (the new Nikon D4S goes up to 409,600...seriously!)

When you point your camera at something, three things are taken into account. The shutter speed, the aperture, and the ISO. These three all interact to bring about a suitable exposure.

The important thing to remember is this: changing the ISO has the same effect as changing the aperture or shutter speed. Changing any one of these elements affects how the camera will analyse the scene in terms of metering (i.e. measuring the light).

If we take some examples, it might be easier to understand.

Let's say you want to take a picture of something. You set the ISO to its base, let's say 100.

You put your camera in aperture priority (where you decide the aperture and the camera decides the shutter speed). You want a shallow depth of field to blur the background, so you set it to 2.8.

You half press the shutter to get a reading. The camera tells you that 1/250 is a suitable shutter speed. You trust the camera, take the picture, and it's fine. The camera was correct. (They usually are, but not always, which you will discover as you keep studying).

Now you do the same thing, but you raise the ISO to 200. You keep the aperture at 2.8. The camera should now tell you that the suitable shutter speed is 1/500. "Hm", you think, "why is that?"

It's because raising the ISO by one unit (called a "stop"), you are now allowing twice as much light into the camera. The camera senses this, and accordingly tells you that you don't need such a quick shutter speed. You raise the ISO from 100 to 200 (MORE light in by one stop), and the camera tells you that you can increase the shutter speed from 1/250 to 1/500 (LESS light in by one stop). This is keeping the balance between the two. One changes and so the other changes to balance the exposure out. If you ignored the camera and still shot at 1/250, your picture would be one full stop brighter because you would be letting in more light than necessary. The result is an overexposed (too light) picture. This can be done for deliberate effect, but again that's something for the future.

The same thing happens with aperture. Taking the same example as above: ISO 100, aperture 2.8, recommended shutter speed 1/250.

Again, you raise the ISO from 100 to 200 but keep the shutter speed at 1/250. Remember what you're doing? You're allowing too much light in. The camera will sense this and tell you that the correct aperture is...(can you work it out?) f2, which is one stop LESS than 2.8. In other words, the hole in the lens is SMALLER.

Why? Because you've told the camera to let in more light, but kept the shutter speed the same. The camera knows that this will give you an overexposed picture, so it tells you to close down the aperture (i.e. make the hole in the lens smaller) so it lets in relatively less light for the duration of the shot. Again, it's balancing everything out. You're telling the camera "let in more light by one stop", so the camera thinks "OK, then I'm going to let in less light by one stop". The result should be a good exposure.

"Also how do you know when to change ISO?"

Generally, you change ISO when the ISO you are at gives you a shutter speed which is too slow or an aperture which is too shallow. Imagine you want to take a shot, but it's getting dark. You put your ISO at 100 and your aperture at 2.8. The camera looks at the darkness and tells you that you need a shutter speed of 1/10, i.e. a tenth of a second. Not many people can get a sharp picture at that speed unless they have a tripod. So unless you have a tripod, you would have to raise your ISO until you got a shutter speed you were happy with.

The same goes for aperture. If you want a shot with a lot of depth of field (i.e. where most of the picture is in focus), and if it's dark, and ISO of 100 will probably mean using a very, very shallow aperture. Probably the lowest that the camera is capable of. This won't give you the depth of field that you want. So again, you raise the ISO until the aperture is appropriate.

Hope that clarified things rather than muddying them!

ISO measures the sensor's sensitivity to light. The higher the number, the more sensitive the sensor is. Doubling of the ISO number makes the sensor twice as sensitive. For example, if the correct exposure is 1/30th at F:5.6 at ISO 400, you could use 1/60th at F:5.6 or 1/30th at F:8 at ISO 800. High ISO is useful for low-light conditions, or when you want to use a fast shutter speed and/or small aperture. The tradeoff is that the sensor will have more noise (random variation in pixel brightness and color) at high ISO. Better quality sensors have less noise: a good DSLR can produce excellent quality images at ISO 800, while inexpensive point&shoot cameras can be noticeably noisy at ISO 400.