Quote:
Originally Posted by Trashcan
So a slower speed (like Velvia) would produce a finer image?
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Not necessarily. Think of ISO as the film's sensitivity to light.
Higher ISO = higher sensitivity = less light required for correct exposure
Your ISO should depend on your lighting conditions and your camera settings. For simplicity's sake, let's just say you're shooting at f/4.
If you're out in broad daylight, you should shoot at the lowest ISO you have. Yes, you can shoot at higher ISO, but the image will likely be more grainy. A shot in good lighting should easily put you at 1/125 or faster. The film doesn't need much light to expose properly. You're used to using 200 because 200 isn't all that grainy when used in daylight, and it allows for a respectable range of shutter speeds in most typical lighting.
Now consider a bar or concert venue. If you tried to take a shot with the same settings as you did out in full light, you'll get a totally underexposed image, or a shutter speed that is so long all you wind up with is a blur. By increasing the ISO, you increase the sensitivity to light, which shortens the amount of time the shutter would need to be open. Yes, the image is a bit more grainy, but it sure beats not having any image at all.
Higher ISO shots can usually be improved using noise reduction programs like Noise Ninja and Neat Image. I use Neat Image, and it's pretty effective.
I generally shoot in aperture priority with my Rebel XT. I'll set my aperture to whatever I want it to be for that particular subject, and based on what I'm shooting, adjust the ISO to get an acceptable shutter speed. For example:
If I'm shooting handheld, I try to get at least 1/125. ISO gets adjusted to allow for that.
If the camera's on a tripod and my subject is stationary, I'll put the ISO to 100 or 200 and let it take however long it needs.