ISO

What it controls:

How sensitive the camera's sensor is to light

What is the range?

Low numbers to high numbers: the higher the number the higher the sensitivity (i.e., the darker the situation a camera can be used in while still having enough light to create an image)

Important issue:

The higher the ISO number used, the 'grainier' the photo looks (that 'grain' is called 'noise'). Below are details from two photos where you can see a comparison of a low ISO (50) on the left, and high ISO (25,600) on the right. Note particularly the blurry green section of both photos: in the ISO 50 photo the green is smooth: in the ISO 25,600 photo the green is noisy. To repeat: the higher the ISO, the noiser the photo.

     ISO 50                                                                                                            ISO 25,600

About:

For those of you who remember film, digital ISO is the exact same thing as film ISO, which comes most commonly in speeds of 100 or 400. Probably the number one advantage of digital over film is the high degree of sensitivity to light that digital is capable of while still looking good: how a camera can handle high ISOs is far more important than megapixels (every camera available today has more than enough megapixels). To change the ISO on your camera, look for either a button or menu that says 'ISO' - if you're not sure where it is, check your instruction manual.

Basic rules:

- Always set your camera to the lowest possible ISO for the light you have (for a better quality image with less noise)

- The more light you have to work with, the lower the ISO you can set your camera to. Conversly, with less light you need a higher ISO

- The higher you set the ISO the lower the quality of the image

In practice:

This is the first thing that needs to be set on your camera before you shoot, and this is based on how much available light you have. ***NOTE: human eyes' ability to see in low light far exceeds what cameras can see. Even a room which seems relatively bright to us can be pitch black to a camera: keep this in mind as it will make how your camera acts make more sense.***

If your camera has an AUTO ISO feature, feel free to use it. AUTO ISO will change the ISO automatically based on how much light you have as measured by your camera's light meter (which was discussed in the 'Light' section above). Pay attention to what it's doing though, so that you start to see what the camera chooses under different light. There will come a point where you'll want to know how to judge this on your own, and there are times that you'll disagree with what your camera wants to use.

If you don't have AUTO ISO on your camera, don't worry about it - here's how to figure out where to set it. Think in terms of ISO 50-200 being used in bright sunlight outdoors, ISO 400-800 being used in cloudy weather or indoors with relatively bright lighting, and ISO 1600+ being used in any low-lighting situations. The best way to learn is to experiment: see what the different ISO settings look like on your camera. As long as you're satisfied with the image quality that you're getting, don't hesitate to use a higher ISO.

NOTE: If you're using a tripod and are shooting something still (landscape, cityscape, still life, etc.) then you can use as low an ISO as you'd like - this will simply affect your exposure time. 

ALSO NOTE: If you're trying to freeze motion in lower light, a tripod is of no help to you as you'll still have to use a higher ISO in order to freeze the action.