Changing the ISO setting on your camera changes the sensitivity to light of the image sensor inside of it.

The lower number that you select the less sensitive the sensor is to light (and conversely the higher the number the more sensitive it becomes).

This is useful when you're shooting in different lighting situations - particularly when there's low light and you might not be able to use a flash (you'd bump up your ISO setting in this case).

The only cost of increasing ISO is that as you do it you'll notice that the 'noise' or 'grain' in your shots also begins to increase.

. Am I hand holding the camera? - when using a tripod you might be able to use a slower shutter speed which would allow you to lower your ISO.

. Is my subject moving? - if your subject is perfectly still (like when shooting a still life) and where you're using a tripod you'll be able to slow your shutter speed and lower ISO.

. Do I need a big Depth of Field? - If you don't need a large depth of field you might be able to increase your aperture which allows more light into the camera and will allow you to lower ISO.

. Can I use some Artificial Light? - using a flash or even switching on a light can help to get more light into your camera - allowing you to decrease your ISO setting.

. Can I get away with Grain? - sometimes a photo can actually look better with grain. Some photographers love the mood and atmosphere that a little noise can add and will bump up their ISO in the hope of getting it.

. How big will the image be enlarged? - the reason that noise is not able to be seen on your camera's LCD is that it is very small. As a result the pixels in the picture are small also and the grain is unable to be seen. It is only when you enlarge the picture on a larger screen or in printing that it becomes noticeable. If you're only ever going to use the shot in a small size you can probably get away with a higher ISO.

Keep in mind that it's only when you shoot in a manual or semi-manual mode that you need to change ISO. When you're in auto mode or one of your camera's preset modes it will select the lowest one that it can for you.

Also keep in mind that if you change ISO that you will need to get in the habit of checking what setting is selected at the start of every photo shoot. Many photographers have been disappointed at the end of a shoot to find that they'd forgotten to check what ISO setting they'd left their camera on in their last shoot. There's nothing worse than thinking you're shooting at an ISO of 100 only to find you forgot to switch it back from 1600.

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