Another related question I had is, do these cameras allow one to vary the shutter speed along with varying the ISO? Or is there always a fixed shutter-speed-to-ISO ratio? That's another thing I don't get. Because it seems like you would want a different shutter speed for "high movement" shots than for dark shots.
And I was confused by CNet's review of the SD800 IS when they said the higher ISO's would be good for "high movement" shots (http://reviews.cnet.com/Canon_PowerShot_SD800_IS/4505-6501_7-32069607.html) - does that mean shots in which the subjects are moving a lot, or shots in which you want a lot of movement in the photo (i.e. blur)?
I'm in the market for a digital camera, and I'm confused about ISO ratings. I'm hoping somebody can clarify this for me:
Basically, it seems the higher the ISO rating, the more sensitive the "film." However, as I understand it, this is used for 2 very different purposes: For low lighting, a high ISO rating can mean that it pulls in more light and gets a better picture, usually in combination with longer shutter times. Whereas for high-action speeds, a high ISO rating can mean that it pulls in more light, allowing LOWER shutter times, so there is less blurring.
Am I right about this or wrong about this? I am super confused. This quote isn't helping either (in a review of the powershot SD900):
"Finally there is the excellent and more versatile Canon Powershot SD800 IS. With its wide-angle lens, the SD800 covers a wider-range of subjects. With its stabilization, the SD800 IS can shoot at lower ISO settings (with lower shutter-speeds) than the SD900 in the same condition. Effectively, this means that under less-than-ideal lighting, the SD900 loses some of its resolution advantage compared to the SD800."
All of this just blows my mind. Can somebody provide me with a simple explanation of how ISO works with digital cameras and how I would choose ISO when I am taking a photo?