WORK WITH THE ISO
Changing the ISO changes the sensitivity of your digital camera's sensor to light; the higher the ISO number, the more sensitive it becomes. This can be very handy if you don't have much available light, or if you are changing from outdoor to indoor lighting. It is also sweet if you want to be more discreet and not use a flash. But, remember that the downside of using higher ISOs is that more digital 'noise' can be introduced into your picture - you will see it as speckled or unwanted grain in the image. Of course, you can also use that to your advantage, for old-time effects, film effects and such. The noise can be especially useful when shooting "grunge" shots for teens and bands. Just remember to check your ISO when you change lighting, so you have a good balance of light, noise and such. One final ISO thought: If you are looking for specific effects, such as extremely deep or shallow depth-of-field, and you cannot achieve what you want by changing the lens/shutter speed settings, change your ISO! You CAN shoot at f1.4 in daylight...you just needed to set your camera properly!
-By Tom McElvy