ISO...

ISO, short for International Standard for Organization, really pronounced "eye so" but more commonly "eye- S- O" ( so much so I personally have never heard it pronounced eye so but who am I to argue with ISO).

In photography, when you talk about ISO you are talking about sensitivity to light, how sensitive to light is your film or digital sensor ? ISO speeds with a lower sensitivity to light such as 100 or 200 mean you need a lot of light to use that setting on your camera and have a correct exposure. You can do this either by shooting where there is lots of light, IE outside on a bright day, or opening your aperture ( IE f2.8, f4) to let in more light or going with a longer/slower shutter speed. ( IE 1/30, 1/15 ). Conversely, ISO of say 400 or 800 lets you shoot in areas of less light, ( maybe the inside of a house) or use a smaller aperture( bigger f number IE f22) or faster speed. So as you can readily see, correct exposure depends on these three factors, ISO, aperture and shutter speed.

So why not just jack up the ISO as high as you can then you can use the smallest aperture and fastest speed on your camera and forget about ever changing it ? Because higher ISO, like everything in life it comes with a price, namely more noise( that speckled, grainy looking stuff you notice in the shadows of that photo you took of Auntie Rose in the darkish living room a few weeks ago). You can use a noise program to reduce the noise( Noiseware Community is a good freebie, other programs such as Noise Ninja are available for reasonable cost. Some editing programs include a noise reduction setting as well.) BUT noise reduction comes at the cost of image detail...so rule of thumb is use the lowest ISO you can.

Cameras today are much better with noise than they used to be ( my Canon Rebel xt was pretty bad at ISO 400, unusable at 800 while my 40d is pretty good up to 1600 when I might need to remove a little noise, might not depending on the situation, some like the Canon 5d and upper line Nikons are even better) so the good news is with a newer camera you might be able to use a higher ISO /higher shutter more routinely..When? Why? Weally? Even on bright days an ISO of 100 or so might not allow me to raise the shutter speed high enough to take a photo of a flower when there is a stiff breeze blowing but a higher ISO will allow me to stop the action. Great!!!

So generalities...use as low ISO as you can go . If your subject is stationary, you don't really need a large dof , have a flash, or want to be able to print out some nice sized photos, a low ISO might be just the ticket. If you have a moving subject, a large dof, using natural lighting, don't mind a little artsy grain in your 8x10 or won't be printing bigger than a snapshot( noise is little and won't show up much in a small photo), move up the ISO....

But don't forget to lower it again next time ..sure sign you forgot is you will have a shutter speed of 1/8000 instead of 1/200...been there ;)