Question about ISO speed

I have a Canon Digital Rebel and have finally decided to begin playing around with the ISO speed. I shoot in RAW most of the time but I don't fully understand when to change the speed. Is it the lower the speed, the more light that is let in? I've always used 400. The family is going to Sea World this weekend and I want to make sure I have the camera set on the right speed to catch any action shots of Shamu plus the silly stuff the kids will do.

Your suggestions would be appreciated!
Thanks
Brandi

The higher the ISO number, the more "sensitive" the camera is to a given light level. That means that, for a given aperture setting, if you need 1/125 second shutter speed at an ISO 100 setting, you can use 1/250 second with an ISO 200 setting.

The "faster" the ISO setting (higher numbers), the more the image quality will suffer. It isn't too bad up to about ISO 400 with that camera, but you will start to see "noise" in your photos at faster ISO settings.

So, for something outdoors with a few possible action shots, would 100 or 200 be best for the ISO?

Thanks
Brandi

If it's a nice day outside, ISO 100 should do you just fine.

You're confusing ISO with shutter speed, I think.

ISO = Sensitivity to light
Aperture (f/stop) = Amount of light allowed to enter the camera
Shutter = Duration of shutter opening

You might consider setting it on Tv and increasing the shutter to capture action. A shutter of 1/500 is the minimum to freeze motion w/o blur. If the light is low when you're shooting, though, it may not work.
If you're just wanting family vacation snapshots, consider shooting in P mode and perhaps even in JPEG to capture the maximum number of shots with a minimum of hassle.

It is best to leave ISO at the highest possible setting - in the case of the Drebel it is 100. And then, after adjusting aperture and find that the shutter speed is too long for a shot, you can adjust (compromise) on the ISO to get a faster shutter speed.

A higher ISO increases the sensitivity of the camera to light and will end up in 'noisier' pictures - which is basically stray light getting captured by the sensor in its over-enthusiasm to capture every bit of light there is

kram wrote:
It is best to leave ISO at the highest possible setting - in the case of the Drebel it is 100. And then, after adjusting aperture and find that the shutter speed is too long for a shot, you can adjust (compromise) on the ISO to get a faster shutter speed.

A higher ISO increases the sensitivity of the camera to light and will end up in 'noisier' pictures - which is basically stray light getting captured by the sensor in its over-enthusiasm to capture every bit of light there is

Actually, I think this gent means the "lowest possible setting." ISO on your camera goes from 100 (least sensitive) to 1600 (most sensitive). As ISO increases you'll find that noise in the photo increases. In general, you want to use the lowest ISO for the lighting conditions in which you're shooting consistent with your shutter speed/aperture needs.

May I suggest an excellent book to help you understand these basic concepts? Try Bryan Petersen's Understanding Exposure or his newer book, Understanding Digital Photography.

Have fun at Sea World,

Mark

Sorry about that - meant 'highest quality setting' = 'lowest possible setting' = ISO 100