ISO... when to use which?

Most of the time I shoot a the lowest ISO setting. That is the best way to get the lowest noise possible. The only times I ever really change it are when I'm shooting in really dark places, or if I'm somewhere semi-dark and I can't use a flash.

well, i only shoot in at the lowest iso setting. i don't change the iso. i would rather use flash or not take the shot than change the iso...:) but that's me... basically, you want to increase you iso when you can't get a good sharp photo (i.e. gettin' blur from hand holding the camera). how much should you increase the iso? well, i would guess until you can get a nice sharp image. :)

It's really a simple decision actually: low ISO produces finer detail, while higher ISO produces more noise, degrading image quality. So always use the lowest ISO you possibly can! Now, what does this mean... Given a certain aperture, what is the longest length of exposure you can live with? If your subject is absolutely still (say, a night *landscape* shot), then even if the exposure required is 30 seconds, using a tripod will make this shot possible even at low ISO. What I'm saying here is that you shouldn't necessarily push up the ISO *because* there is less light. The reason you would push up the ISO is only because you absolutely need to have a shorter exposure. When is that? One example is when your subject is not still. No matter how still your camera can be on a tripod, it won't help to freeze the action. For example, people are not rocks: you probably don't want to go longer than something like ~1/30th even if the person is "idle". For moving targets, you'd have to go much faster than that -- sports photography, for example, usually requires speeds in the thousandths!) Another example is when you want to hand-hold the camera. Then, at some point, camera shake will become a problem, because no matter how zen you are, you are not as still as a tripod. The general rule of thumb is not to go slower than 1/focal (that is: 1/50th on a normal 50mm lens, 1/300th on a telephoto 300mm lens, etc.) To help you with hand holding, many lenses or camera bodies today use image stabilisation mechanisms. You can get lenses today that promise to help you get up to 4 stops slower shots than you could normally hold without causing noticable camera shake! (For example, if you're using a 200mm lens, say, a 3 stops of image stabilisation would allow you to take a sharp shot at 1/25th !) Now, whatever happens, unless you're making pictures for a client in circumstances where quality is a real issue, don't worry too much about added noise at higher ISOs. It really only shows when you're looking at your picture too close (such as "pixel peeping" images at 100%) or making huge prints. If you're only printing small pictures (say, 8"x10") or only looking at your pictures on your computer screen, it really doesn't matter all that much. Take the shot and stop worrying about it. Consider that today's digital cameras have impressively low noise at high ISOs, compared to the grain you would get in the film days -- old school photographers can't believe how clean images are today! If you really need lower ISOs that you can't achieve today, then either produce more light or buy faster lenses. The maximum aperture of a lens is one of its defining caracteristics, and is often responsible for it's price for the most part. Hope this helps...