What is ISO and Noise?
I guess I am in a mood to do demo's today!
ISO settings are simply a measurement of how sensitive your sensor (or film) is to light. The HIGHER the setting the more sensitive your sensor is, but at a cost of grain or noise, especially in the shadow areas. If you set a lower ISO, then you will have less noise, but you will need a larger aperture, or a longer shutter speed.
There are times where you HAVE to set a very high ISO in order to get a useable picture. Examples may be things like high school gyms, wedding ceremonies (where flash is not allowed), or even in your living room. However as a general rule, you will want to select the lowest possible shutter speed that provides you a useable shutter speed.
Getting back to Reef tank photography, depending on the lighting of your tank, and what you are intending to take pictures of, (fish or corals) you will find that ISO 400 is usually plenty fast. If you are taking coral pictures with a tripod, then I would set your camera to the LOWEST ISO number you can, as it will give you the least noise, and best color.
Color is another thing that is affected by ISO, the higher the ISO the duller the color, the lower the ISO the more saturated the color is.
Okay... I guess I have typed enough, on to the PICTURES!!!! ISO 100 ISO 200 ISO 400 ISO 800 ISO 1600 ISO 3200
Now, NO ONE mention the Valonia
but that is actually where the noise is most evident, the shadow area. As the ISO increases, do you see the "grain" or color flecks increase in those shadow areas? That is the NOISE.
Remember, noise is not the end of the world, it can be reduced in post processing, or even eliminated totally with programs like Noise Ninja. If you ever find yourself in a place where you have to increase the ISO to get a clear image, then by all means DO IT!! It is much better to have an image that is in focus with a little grain, then it is to have a blurry image from either camera shake or your subjects movement. (Bird in hand is better then 2 in the bush mentality.)
I hope you find this information useful, and by all means if you have any questions, post them here!