The ISO (International Standard Organisation) or the ASA (American Standard Association) as it was formly called, is the standard for expressing the sensitivity of the sensor or the film to light. For a point and shoot camera this is normally 50 – 100. Digital cameras are more sensitive to light then film or fixed lens cameras, so normally the slowest ISO is often 200 or 100.
The ISO is expressed as film speed. The higher the number, the more sensitive or faster the film. Film with lower sensitivity to light, (a lower ISO) requires a longer exposure and is called a slow film. A film with hight sensitivity (a higher ISO) can shoot the same scene with a shorter exposure, or shutter speed and is called a fast film. Now, of course if you are shooting with a digital camera, you have that advantage that you can change the ISO for each image. With film photography, the whole film is the same ISO!
When shooting with shutter priority, the camera chooses the shutter speed for proper exposure and this will depend on the lighting, the film speed (ISO) and the aperture. A higher ISO, allows it to be exposed faster, so the shutter does not have to stay open as long for the same size aperture.
With a digital camera, when the ISO is increased, it amplifies the signal from the sensor. Each time the ISO is doubled, the camera can use a shutter speed twice as fast for proper exposure in the same lighting, at the same aperture. This I think is amazing! It was all so confusing to me at the start, but when I look at it now, it somewhat makes sense! I control the amount of light entering the sensor and I have 4 things at my disposal to use to alter the lighting.
Anyways, getting back to ISO….. the only down side of using a faster ISO is that this amplification can cause noise, or film grain so the photo will turn out bad quality. Again, this is an effect that the photographer may be wanting to create on purpose!
But for good quality photographs that can be enlarged and still clear and sharp, it is better to keep the ISO speeds set lower for a cleaner image. My camera ISO starts at 200 and goes to 1600.
You may not see the need to alter your ISO, but it is good to know the effect it will have on your images. A perfect example of needing my ISO altered, happened to me a few weeks ago. I was shooting a wedding. It was a beautiful day, the photos outside i could shoot away at an ISO of 200. But inside the church, was a different story. It was dark, and dimly lit! I wanted clear sharp images. I had a tripod, but didnt want to be using my flash during the ceremony. I needed to increase my ISO to 1600 to get clear bright images in this dark environment. Increasing your ISO increases your cameras sensitivity to light. So you can get the image you want, even if you only have a little available light.
I hope that has explained ISO to you clearly! Now get out and experiment!!
Good luck - Rebecca!