Read about shutter speed and aperture first.

This guide will be on the effect of different ISO settings on the camera. For once there are no silly markings or trying to understand fractions, ISO is simply denoted by a number. 100 is the lowest you can go, around 1600/3200 is the highest. Some far more expensive cameras can go higher.
The range goes like this:
100, 200. 400, 800 and 1600.

The ISO rating basically tells the camera how sensitive it needs to be to the light that hits the sensor. For example, if you're taking pictures during a sunny day I would expect 100-200 to be adequate as there is plenty of available light. If you're indoors or in dim situations you may have to resort to using 400, 800 or 1600.
Happily the scale is pretty simple to understand, every time you go up a level, the camera is twice as sensitive.
400 ISO is twice as sensitive as 200
800 ISO is four times as sensitive as 200 etc.

Camera Noise
Being able to control light sensitivity is an excellent feature as it can allow you to get faster shutter speeds in poor light conditions, or a smaller aperture of have a larger depth of field. However, as always, there is a trade off. The higher the ISO, the more noise there is in the picture.
'Noise' is a term used to describe little dots that appear in pictures when using high ISO and massively degrade the image quality.
Here is an excellent example. Notice the grainy affect all over the picture, ruining the clearness of the sky etc.

In most situations using ISO 100, 200 and 400 have no noise, or very little, and can create very nice, clear images. What you have to decide is what's most important for your shot, perfect image quality or a fast shutter speed.

Practical Uses
So, you've read through my three guides, tried to digest it and maybe had a go at playing with the settings. What I thought I'd do is pick a couple different scenarios, suggest some settings and explain why I chose them. Hopefully thinking about it practically will help some of the information sink in.


Scenario 1
You're a landscape photographer. In front of you is a magnificent display of trees and hills spanning to the horizon. The focal length of the lens is 25mm.
Here you'll want to capture as much detail as you can. As this is the most important factor, I'll chose the aperture size first. A very small aperture size is needed to create a large depth of field, so I'll suggest F20.
Next chose your shutter speed. So you can use a low ISO (to prevent noise coming into the shot) you will need a long exposure time to compensate for the narrow aperture. I'll suggest around 1/10.
If you have a tripod that will help massively as you can put the camera on there and not worry about camera shake spoiling the detail. If you don't have one then chose at least 1/focal length, I'll chose 1/30 to be on the safe side, and increase the ISO to compensate for less light. I expect that 200/400 will work.

Scenario 2
You're taking pictures of an indoor 5 a side football match. The light inside the hall isn't very good.
For sports events, freezing the action in the picture and preventing blur is very important. Because of this I'll chose my shutter speed first and suggest 1/500. Because of the low light we will need the largest aperture possible to compensate for the fast shutter speed, so F3.5.
Because the shutter speed is so fast I doubt you will have enough light to expose the image correctly so we need a high ISO, so I'll pick 800. There may be some noise in the picture because of it but you'll have to deal with it. Slightly noisy is better than a massively underexposed image.

As always, please post up any questions

i couldnt be bothered to read that.. i just looked at the picture


ho w rude of me! but i like to comment anyway lol

It's a very good guide! Sit down with your camera and just spend 10mins and have a quick read

Someone at least opened it up to comment Cheers Micky